Understanding Crop Factor


There is a great deal of confusion surrounding crop factor, and it is particularly difficult to explain, but let’s give it a try, shall we?

Before we dive in, let me dispel two vicious rumors related to crop factor that are circulating through the photography (Internet) world today:

  1. Crop factor does NOT affect a lens’s focal length.
  2. Crop factor does NOT affect a lens’s aperture.

Before you scroll to the bottom of the screen to leave a comment to the contrary, let me explain why I am stating these facts…

Focal Length The focal length of a lens, expressed in millimeters, is the distance along the lens’s optically central axis (beginning at the rear nodal point) to the image plane in the camera (often illustrated by a "Φ" on the top plate of a camera body) when the lens is focused at infinity. The image plane in the camera is where you will find your digital sensor or film plate.

Therefore, a 50mm lens can measure 50mm from the point where light rays begin to exit the lens in the same direction as they entered the lens until they arrive on the image plane. Some “pancake” lenses and mirror lenses have optical tricks to shorten them, but in general, the focal length is that physical measurement.

A zoom lens can change the physical focal length of a lens. Sometimes this movement is contained inside the lens—the lens body does not physically change length—and other times the lens does change its size.

However, regardless of what kind of camera or sensor you place behind the lens, the focal length will not change just because you have a larger or smaller sensor or frame of film. I will explain later how sensor size (or film size) changes the equivalent focal length—not the true focal length of the lens.

Aperture  is the size of the opening in the lens. Some lenses have fixed apertures that cannot be changed, but most photographic lenses have variable apertures to control the amount of light entering the lens. This opening is regulated by a diaphragm comprising blades that can be adjusted to vary the size of the hole (aperture) through which the light passes.

In photography, aperture is expressed as a ratio of the focal length to the diameter of the aperture opening. The ratio is commonly referred to as an f/number, f/stop, focal ratio, f/ratio, or relative aperture.

This ratio is based on physical measurements and is completely independent of the size of the camera’s sensor or the size of the film you are shooting. Sensor size has an effect on depth of field, but not because it changes aperture. Aperture is independent of film frame or sensor size.

35mm Format

The first thing to know about crop factor is that, as with all “factors,” we need to have a base reference from which to work. In the photography world, this reference is a piece of 135 film. In the digital photography world, “full-frame” sensors are the same size as this film; a film frame with a width of 35mm. Cameras of this photography format are collectively known as “35mm cameras.”

A 35mm film strip measures 35mm across

One source of crop factor confusion is the use of “35mm” when discussing the reference. The value in this case is used not as a focal-length measurement, but as a measurement of the dimensions the frame of film. The film image area measures 24 x 36mm, but the strip is 35mm wide. So, when you think of “35mm” when it is used in reference to film or the size of a camera sensor, know that you are not referring to lens focal length. You can mount a lens of any focal length, even a 35mm lens, on a 35mm camera. The focal length is the focal length. Film and sensor dimensions are different.

For years, the 35mm camera has been, by far, the world’s most popular camera format. Because of this, for those of us who grew up in the world of 35mm cameras, when we think of the field of view given by a lens of a certain focal length, we can visualize what the photograph should look like. In the 35mm camera world, a lens with a focal length of around 50mm will provide a “normal” view with its human-eye-like field of view. Lenses with shorter focal lengths will provide a wider view and lenses with longer focal lengths will provide narrower or telephoto views.

Digital Sensors

Life was simple back when almost everyone was shooting 35mm cameras and 35mm film. Sure, there were those making magic with medium format and large format cameras, and there were point-and-shoot cameras that took specially made smaller films. My first camera, handed down from my grandmother, was a Kodak Instamatic 30, with its 13 x 17mm 110 film. Back then, no one really paid attention to “crop factor,” even though it existed. I’d bet most photographers didn’t know the dimensions of their 110 film, nor did they know the focal length of the tiny lenses! You just looked through the camera and took the picture it gave you.

Then, digital photography arrived. In its early days, most sensors were smaller than 35mm film, and a virtual can of worms was opened. Why? Because the sensors were smaller than 35mm film, the images seen through a lens of any particular focal length had a different field of view than that of the same lens on a 35mm film camera. Suddenly, a 50mm lens no longer had a “normal” field of view; it was a bit more of a telephoto.

The cropped sensor “sees” a narrower field of view

If you never shot 35mm film, this was no big deal because your mind’s eye did not have a 35mm film reference for different lenses. But photographers entering digital imaging decided that they needed to know the “35mm equivalent” field of view of various lenses when attached to a camera with a digital sensor smaller than 35mm film. The reality of it is, “crop factor” serves to translate a measurement into a language in which many of today’s photographers were never fluent to begin with. And, because of this, many of you out there have been very confused and frustrated by the mention of crop factor. Hopefully this article will end your confusion!

Crop Factor

A round lens produces a circular image circle—not rectangular. The sensor, or film, at the back of the camera captures a rectangular portion of this image circle. When we use 35mm film as a standard, any camera with a sensor smaller than a frame of 35mm film will cover a smaller portion of the image circle produced by a given lens and will thereby change the field of view of that lens. This is the “crop” part of the crop factor.

However, because traditionally, the field of view produced by a given lens has been described not as a measurement of degrees, but by the focal length (kind of the “name”) of the lens, we need to translate the cropped field of view into an equivalent lens focal length.

For example, if you attach a 50mm lens to a camera with a smaller-than-35mm film sensor, you will have to multiply the focal length of that 50mm lens by a factor derived from the size differential of the sensor to calculate the 35mm equivalent focal length. This will then give you the means to figure out the lens’s field of view based on that new equivalent focal length. This is the “factor” part of crop factor.

This multiplication factor is the ratio of the size of the digital sensor to the dimensions of the 35mm film negative.

Formula: The diagonal of a rectangle can be determined by a2+ b2 = c2

Full Frame: 24mm2 + 36mm2 = c2

576 +1296 = 1872

Square root of 1872 = 43.3mm

Full-Frame or 35mm Diagonal / Crop Sensor Diagonal = Crop Factor

So, if you have a camera with an APS-C-sized sensor (circa 15.6 x 23.5mm or 14.8 x 22.2 on Canon), plug in the numbers and you will get a crop factor of 1.5x (or 1.6x for Canon).

Then, to find the equivalent focal length of the new field of view afforded by the smaller APS-C sensor, multiply the true focal length of the lens by 1.5x to get the 35mm equivalent focal length of the lens. A 50mm lens on a camera with a 1.5x crop factor APS-C sensor gives a field of view equivalent to that of a 75mm lens on a full-frame or 35mm film camera.

Remember, the actual focal length of the lens is unchanged, as is its aperture.

In our example, if you weren’t familiar with a 50mm lens’s field of view in the first place, this doesn’t really matter. But if you were familiar with the 50mm lens’s field of view, you will know that this same lens, when placed in front of the smaller sensor, has a narrower field of view than your normal vision has.

A comparison of relative sensor sizes.


If you have a zoom lens on a smaller-than-full-frame camera, you can figure out the effective focal-length equivalent by multiplying both focal length numbers by the crop factor. For example, a 70-200mm lens becomes a virtual 105-300mm lens on a 1.5x APS-C sensor.

Cameras with sensors or films larger than a 35mm frame will have sub-one crop factors. For instance, a medium-format Pentax 645Z’s sensor measures 33 x 44mm. This gives it a crop factor of 0.78x. A 50mm lens on this Pentax camera gives an equivalent field of view of a 39mm lens.

Full-frame versus The Rest

The crop factor discussion inevitably leads us to the full-frame versus smaller-sensor debate. For my take, click here.

So as not to drive down the well-trodden path here, in summary, full-frame cameras are ideal for landscape images because there is no crop factor and wide-angle lenses maintain their wide-angle field of view. Smaller-sensor cameras give lenses a virtual telephoto effect that is ideal for some sports, wildlife, and macro work. Both formats have advantages and disadvantages.

Another thing to mention: there are “regular” lenses and there are lenses specifically designed to operate on smaller-sensor cameras. These small-sensor lenses may not work well on their full-frame cousins. On a 35mm film or full-frame digital camera, you may experience heavy vignetting. If the small-sensor lens does work on a full-frame digital camera, the camera might simulate the smaller sensor the lens was designed for and automatically provide the crop factor field of view. A regular lens will work happily on a full-frame digital, 35mm film, or smaller-sensor camera. The crop factor will only apply to a lens if it is used on a small-sensor camera. Today, some manufacturers refer to their “regular” lenses as “full-frame lenses” to emphasize that they are not designed specifically for smaller-sensor cameras. But, before digital photography, all 35mm format lenses were “full-frame.”

The Final Word

Crop factor is really quite simple. The confusing thing is that, as I said earlier, it exists to translate an angular measurement (degrees of field of view) virtually into a linear measurement (millimeters of lens focal length) so that old-school 35mm photographers can figure out the real field of view of a lens based on an equivalent focal length resulting from using sensors smaller than 35mm film. Get it? Got it. Good!

I suppose that is useful in many ways, but I have seen many frustrated photographers over the years try to understand and explain this concept. Sprinkle in some bogus information on the Web about magically changing focal lengths and apertures, and everything has become a mess!

I hope this has cleared things up for those who are new to photography or who were confused a few minutes ago. If not, I stand by to take your questions! And, in case you were wondering, the Instamatic 110 film camera has a crop factor of 2x. 

For more information about the theory behind crop factor, be sure to watch this engrossing video.



OK, long wanted answer to a question..... So if a ff 50mm lens is on a ff camera... its a 50mm true lens...... if you buy an apsc camera (say a6400 Sony) and an apsc lens labeled 50mm.... is the apsc lens a true 50mm as well? and will both images from both cameras take the same 50mm image?  Or is a 50mm apsc lens still a 75mm on a apsc camera. Thanks 

Hey Aussie J,

I hope I can end your long wait for an answer!

So, a 50mm lens is a 50mm lens regardless of what size sensor it sits in front of...or if it is mounted on a camera at all. A 50mm lens cannot be anything but a 50mm lens.

So, on a full-frame camera, the 50mm lens gives an angle of view that gets associated with a classic 50mm lens. If you put a 50mm lens or an APS-C-specific 50mm lens in front of an APS-C sensor, you will get an angle of view similar to that of a 75mm lens on a full-frame camera. The APS-C camera's angle of view will be different than the full frame camera with any 50mm lens mounted...even an APS-C 50mm lens.

If you want to simulate the field of view of a 50mm lens on a full-frame camera, you have to use a 35mm (focal length) lens on the APS-C camera.

I hope this answers your question, Aussie!

G'day and thanks for reading Explora!

Hi Todd -

   So if I use a Nikon AF-S DX NIKKOR 35mm f/1.8G Lens on a APS-C camera (D500) and take the same image with a Nikon AF-S NIKKOR 35mm f/1.4G Lens on the same APS-C camera (assuming I use the same f stop for both pictures) I should get an image that looks the same?  I think I understand the crop factor scenario, I just get confused with selecting a DX vs Full Frame lenses....If I'm understanding things correctly, they should both perform the same with regard to field of view....my understanding is the primary differences between the two lenses are full frame uses better glass and tend to weigh more because they are built to cover the bigger sensor size.  I hope I didn't add to the confusion.



Hi Douglas,

Yes, you will get an almost-identical image from the DX 35 and the non-DX 35mm lens assuming the camera has not moved and the aperture is the same.

Remember, there are only DX lenses and non-DX lenses. The advantage to the DX lens is that it can be smaller and lighter as it does not need to produce an image circle as large as the non-DX lenses. But, focal length is focal length...a 50mm lens is a 50mm lens no matter what camera you put it in front of.

I hope I didn't add to the confusion either! :) Let me know if you have more follow-up questions.

Todd - thanks very much for your response and for confirming what I thought to be the case.  Your article on understanding crop factor was really helpful to me....and not confusing at all like many of the other explanations I've read.  I really appreciate such a clear explanation of an otherwise confusing subject.  Thanks again!


No worries, Doug! I am glad to help clear up the muddy waters of the interweb! :)  Please let me know if you have any other questions and thanks for reading Explora!

Hey Todd,

I would like to discuss the following scenarios:

I shoot pic 1 using a 50mm f2 lens on a full frame camera for a peach 10 meters away.

Then I shoot pic 2 using the same 50mm f2 lens on an APS-C camera for the same peach 10 meters away. Now pic 1 should have a bigger field of view than pic 2. But if we crop pic 1 to the same field of view of pic 2, they should be exactly the same in terms of exposure and depth of field. The reason being the second shoot has the exact condition of the first shoot, only the APS-C sensor captures a smaller view of field. They should have the same light energy per unit area.

To be able to capture the same field of view of pic 1 using an APS-C camera and 50mm f2 lens, I need to move away from the subject. Say I move away to 15 meters toward the peach. Now I take the 3rd shoot, producing pic 3.

Pic 3 has the same field of view as pic 1, but because the distance has changed, the depth of field definitely changed between pic 1 and 3. Now the question is, does pic 3 have the same exposure condition comparing to pic 1? Since pic 1 is taken at 10 meters away and pic 3 taken 15 meters away, the light energy per unit area reaching the lens must be different. I think because of that, the light energy per unit area reaching the sensor would be different too. Like, if we have a square meter of solar panel, having it on Mercury would capture more light than having it on Earth. When further away, we need a bigger aperture to capture the same amount of light than when we are closer.

So in all, sensor size should affect exposure, if we would like to capture the same field of view as a full frame, right?

Thank you!

Hi Feng,
Great question(s)!
Scenario 1: Full frame, 50mm, subject-lens distance 10m.
Scenario 2: APS-C, 50mm, subject-to-lens distance 10m.
Yes, Photo 1 will have a wider field of view.
When you crop in, the exposure will be the same, but the depth of field will be slightly different (very slightly) due to the change in reproduction ratios. It might not be visually noticeable, but, mathematically, it happens.
Scenario 3: APS-C, 50mm, subject-to-lens distance increased to simulate full-frame field of view.
Different exposure? In theory, maybe. Your question actually was very thought-provoking and I had to do some research. And, there is a lot of conflicting information out there!
As you are alluding to, transmitted light follows the inverse square law…when transmitted from a point source radiating in all directions—a light bulb without a shade or, the sun.
If your light source is the sun, the peach will be pretty much the same brightness regardless of your distance from it. This is why mountains, buildings, etc. are not noticeably darker as you get further away from them. If this was the case, a photo of objects on the horizon would require lots of exposure compensation whereas objects in the foreground would be relatively bright—think of photographing down a road that stretches to the horizon—the pavement in the foreground isn’t noticeably brighter than the distant background road surface. Outdoors, unless in shadow, the sun pretty much evenly lights the world when overhead.
If the peach is illuminated by a spot light, it will remain virtually the same brightness regardless of the distance. It reflects the same amount of light regardless of the distance to the camera. As you move further away, the peach gets smaller, but not noticeably dimmer.
If the peach is illuminated solely by a light mounted on the camera, you will see a more profound effect in darkening as you move away from the peach due to the inverse square law and the fact that the distance between the light source and the peach is changing.
A good example of this in a different scenario:
Imagine a street with a tiny house placed at one end. It is nighttime and the house has one window and a light bulb hangs in the center of the room and the light from that bulb and room shine through the window.Regardless of the distance from that window, when you look at that window (or light bulb), the brightness is fairly consistent (unless you get a great distance away). The window does not change its distance from the light source as you move away. Its illumination is fairly constant. Now, think of taking a photo of an object outside of that window using the light from the window as the light source. The further you get from the window, the darker the object is because of its distance from the light source.
The inverse square law applies, kind of. It is a very specific law to a specific type of light source. In the real world we have light reflecting from all over out of doors, or in a studio, we control all the light.
I could meander around a bit, but I do think I knew what you were getting at…does a smaller sensor mean that there is a change in exposure? And, the answer is that, if there is a change in brightness, it is so minuscule that it is not measurable by a camera meter. This is why handheld exposure meters do not have settings for sensor or film size nor is there a separate set of exposure math for smaller (or larger) sensors.
I hope this helps!
Let me know if you have more questions.
Thanks for reading Explora!

Hi, can you suggest me a DSLR? I'm 16 and just starting photography. I have a great interest in photography. I'm confused between the wide range of DSLRs. I've used shoot and point cameras earlier but now I want to upgre? 

Hi alax!

Thanks for writing in! For a beginner DSLR and some other thoughts, check out this article: https://www.bhphotovideo.com/explora/photography/buying-guide/what-is-the-best-camera-for-beginners

Inside the article is a list of beginner DSLR cameras, but also some thoughts about other cameras that might work well for you.

Please let me know if you have questions after reading the piece.

Thanks for stopping by!

If you are looking for a recommendation for a entry-level DSLR camera that would be a good option for your first DSLR camera purchase, I would recommend the Nikon D3500 DSLR Camera with 18-55mm Lens, B&H # NID35001855B, as a good option for your usage needs.  The camera has full automatic control so you can begin using the camera as you would with any point-and-shoot camera, but as you learn more about photography, you would be able to use the camera in full manual mode, taking full control over all exposure settings and giving you full creative control.  The camera has an easy-to-navigate menu system, has a good five (5) frames-per-second continuous burst rate, HD video recording capability, and has a large library of lenses that are compatible for use with the camera.  While it is a DSLR camera, it is one of the smallest and lightest options currently available.  It also has built-in wireless Blutetooth capabilities which would allow you to wirelessly pair your Bluetooth-enabled smartphone or tablet to transfer images from your camera to your smartphone, and/or to wirelessly control your camera.  It would be a good and flexible first DSLR camera purchase.  If you have any other inquiries, you may contact us directly at askbh@bhphoto.com

I absolutely agree with the camera body choice. I myself am a Nikon user and my first camera was D3200. Excellent value for money! I guess that D3500, as its successor, is also a great choice for a beginner.

However, having this experience, if I were buying my first kit today, I would opt for a lens which could provide me a bit more range, for example 18-105mm. Same as with 18-55, you won't get the best quality in the world, but it is a budget option which you could use until you decide if you really want to do photography and to start investing more. It will give you a wide-angle capability, but also a larger telephoto focal lenghts, so you can experiment more.

When you find out which focal lenghts (or ranges of lenghts) you use the most, consider investing in a higher-quality (and more expensive) lens(es) which will give you much better quality. But whatever you do, try to avoid getting G.A.S. (Gear Acquisition Syndrome) and try to invest more in your education and in learning the techniques than in your equipment.

Hi Vladimir!

Solid advice there! Thanks for stopping by and sharing your experience!

after 50 years behind a camera ( past 20 in digital) Still have one question of all this . does the crop factor affect the bokah of an image , lets say wide open (1.8)?

Hi charlie,

Great question!

The answer is: No, bokeh is not effected by sensor size.

Why isn't bokeh effected? Because a lens renders out-of-focus areas the same regardless of what sensor is placed behind it. Remember, a crop sensor is virtually cropping an existing image so the bokeh from the center of a full-frame image will be identical when you remove the edges of said image.

Here is my treatise on bokeh: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/understanding-bokeh

However, depth of field is effected by sensor size. If you want to dig into that math and the myths surrounding it, check out this article: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/depth-field-part-3. What you'll gather there is that if you are shooting both full frame and crop sensor cameras simultaneously (super common) and trying to make the images look the same, you will have different depths of field and, therefore, a change in bokeh.

Having said that...bokeh is bokeh. If you are taking a head-and-shoulders portrait with a 50mm f/1.8 lens on a DX camera and then you switch your lens to a full frame camera to take the same head-and-shoulders portrait, the bokeh will be different in each photo. Why? Because the DOF has changed due to a change in the subject-to-lens distance and the enlargement factor. But, the way the lens renders out-of-focus highlights is identical regardless of what camera it is mounted on.

That got long-winded. Sorry! Let me know if you have more questions. Hopefully I answered your query without adding confusion!


Hi Todd, 

I really appreciate your effort of trying to clear all the confusions of the readers. I have been using Sony E and FE for quite sometimes. But I'm still quite confused when it comes to the question: is the FOV that is viewed from a crop lens on a crop-sensor camera the same as a full-frame lens on a full-frame camera. 

In my experience, I will take an example like this:

1) Crop: Sony a6300 + Crop 50mm f/1.8  (SEL50F18)

2) Fullframe: Sony A7iii + FF 50mm f/1.8 (SEL50F18F)

(Please note: two lenses are different) 

And my question is: are pictures taken by both combos will look the same in term of FOV? (Please disregard about image quality)

Thank you so much for your time!

Hi Long!

Good question. Remember the focal length of a lens is not affected by crop factor. A 50mm lens is a 50mm lens regardless of what camera you stick it on...or if it is sitting in a box on a shelf. It was born a 50mm lens and will go to the dump or crusher as a 50mm lens.

So, even with those two different lenses, when placed before a full-frame and APS-C sensor, you will get a different field of view. The 50mm lens on the APS-C camera will have a narrower FOV than the 50mm lens on a full-frame camera.

Standing in the same place and pointing each camera in the same direction, you will not get the same photo. If you put a 35mm (focal length) lens on the APS-C camera and shot it in the same spot as the 50mm on the full-frame, you would get very similar FOVs.

I hope this clears things up for your, Long!

Let me know if you have more questions and thanks for reading Explora!

I'm so glad that this question was asked because trying to find the answer has been driving me crazy for a long time!  So to make sure I understand your response Todd, the crop factor for the FOV still applies for an aps-c lens on an aps-c body.  The aps-c lens of 50mm on the aps-c camera will provide a FOV that would be the equivalent of 75mm lens on a full frame camera.  Am I understanding this correctly?   Finally knowing the answer to this question will help me sleep better at night!  :)

Hi Anita,

You have your interpretation correct! Sleep well and let me know if you have other questions!

I know this is way beyond what most people want to know, but I love the science of photography as much as I enjoy creating photos.  So, to follow up on Peter D’s question about f stops, if a crop sensor camera’s owner adjust the f stops letting in more light (by multiplying the f stop used on a full frame camera by the crop factor to get the same depth of field as the full frame camera), will this compensate for the reduced total quantity of light a crop sensor camera gets due to it’s smaller sensor size?  Would this give (approximately) the crop sensor camera about the same amount of noise as the full frame camera with the larger light gathering sensor?

Hey Jim,

Those are good questions and similar to Pete's and...the answer is still "No!" :)

It is easy to see why there is a popular opinion that a smaller sensor gets less light than a larger sensor and, therefore, exposure must be different.

There are some that say that a photograph taken at 1/500th and f/8 and ISO200 on a crop sensor camera will be darker than the same photo taken with a full-frame camera at the same moment, but, if that was really the case, companies that made light meters would have to have a "crop sensor adjustment" dial on their meters. And, if you have ever used a hand-held light meter, there is no such adjustment, nor do they sell light meters that are "calibrated" for different sized sensors or film.

The amount of photons of light that pass through the lens aperture diaphragm is the same for a given f/stop regardless of the size of the sensor sitting behind it.

Tiny people at the beach can get the same exact sunburn as giant people. Right?

Sensor noise is a function of a lot of different things—not just the amount of light hitting the sensor. Temperature, humidity, electronic functions, and software all have a play in that. Larger pixels and sensors, in general, dissipate heat better than smaller sensors and pixels, so their digital noise performance is a little better. That isn't really a function of the amount of light hitting the sensor.

I hope this answers your questions! Standing by for follow-ups.

Todd Vorenkamp wrote:

Hey Jim,

Those are good questions and similar to Pete's and...the answer is still "No!" :)

It is easy to see why there is a popular opinion that a smaller sensor gets less light than a larger sensor and, therefore, exposure must be different.

There are some that say that a photograph taken at 1/500th and f/8 and ISO200 on a crop sensor camera will be darker than the same photo taken with a full-frame camera at the same moment, but, if that was really the case, companies that made light meters would have to have a "crop sensor adjustment" dial on their meters. And, if you have ever used a hand-held light meter, there is no such adjustment, nor do they sell light meters that are "calibrated" for different sized sensors or film.

The amount of photons of light that pass through the lens aperture diaphragm is the same for a given f/stop regardless of the size of the sensor sitting behind it.

Tiny people at the beach can get the same exact sunburn as giant people. Right?

Sensor noise is a function of a lot of different things—not just the amount of light hitting the sensor. Temperature, humidity, electronic functions, and software all have a play in that. Larger pixels and sensors, in general, dissipate heat better than smaller sensors and pixels, so their digital noise performance is a little better. That isn't really a function of the amount of light hitting the sensor.

I hope this answers your questions! Standing by for follow-ups.

Hi Todd,

Thank you very much for your article! I am new to photography, and have been trying to understand the main benefits of Full Frame compared to APS-C sensors (I currently have an APS-C and have been thinking of getting a Full Frame); it's been somewhat of a rabbit hole as different sources have been saying conflicting things.

I have heard two key benefits of Full Frame cameras compared to the rest are that 1) Full Frames afford a shallower depth of field (e.g. from a depth of field perspective, a Nikon APS-C camera that is set to F1.8 is actually equivalent to a F/2.7 on a Full Frame), and 2) Full Frames have better quality in low-light/in general as they allow more overall photos to hit the sensor. 

I've read through your answers, and it sounds like neither of the above 2 "benefits" are really true (i.e. F1.8 on a Full Frame and Crop Sensor should yield the same depth of field results, and photons that pass through the lens will be same on Full Fame and Crop Sensor); and your answers make sense to me. 

Based on your article and previous response, it appears the main benefit of Full Frame sensors are broader field of view, and potentially better quality photos as the digital noise performance is better due to larger pixels and heat dissipation. Is this correct, and are there other key advantages of Full Frame sensors?

Thank you for your time!

All the best,


Hi Thy,

You are very welcome! Thank you for the thank you!

I am sorry that you have the same dilemma that many photographers are faced with today.

A couple of corrections for you:

1) Depth of field does change with a smaller (or larger) sensor in front of a given aperture lens. Full frame cameras will allow a shallower depth of field for a given aperture than a crop sensor. However, in my opinion, this difference is not substantial.

2) Low light performance and image quality might be better on a full-frame sensor, but, again, in my opinion, this difference is also not substantial. People quickly forget that almost every professional digital photographer used crop sensor cameras for years and no one that I know of was telling the world that the images were substandard because of the sensor size.

I got my master's degree using an APS-C sensor camera and I shoot jobs for clients with an APS-C sensor camera. Never have I had a teacher or client tell me that I needed a better or different camera.

So, both "benefits" do exist. I just depends on how much you believe these things are important.

Yes, full frame gives you a wider field of view for a given focal length...the "true" traditional field of view that you may or may not be familiar with if you shot 35mm film. You gain a shallower DOF and you may have very slightly better image quality and noise performance. Those are the advantages of full frame, but I do not think they are significant advantages.

Ask your full-frame shooting buddies this question: "If you think a bigger sensor makes better photos, why are you not shooting medium or large format digital?"

Let me know if you have more questions, Thy! Thanks for stopping by!


Hey Todd, thanks for the article, and for cultivating the conversation about it, especially after such a long time since the original post.

One question regarding item 1) in your reply, DoF changing with sensor size: isn't DoF a physical factor of the lens, just like aperture and focal length?

From a purely physical perspective, the image's DoF would not change based on the size of the sensor. However, if you reframe the image to compensate for the different size sensor by changing the distance to subject, then the DoF *would* obviously change, since you'd be at a different part of the focus range. 

The only other potential factor would be the relative difference in the size of the circle of confusion between the two sensors. As a factor of minimum CoC to sensor size, the lens would have to be correspondingly sharper for the smaller sensor to match the sharpness of the image from the larger sensor, but relative DoF in that case would also not change.

One of the biggest struggles I've seen trying to explain crop factor is the unnecessary addition of variables to the mix, and the ensuing confusion they cause. It can be especially difficult maintaining a useful distinction between the *technical* details of crop factor (and all that pesky math) and the application of crop factor to *creative* decisions regarding choice of lens and how it affects the final image. 

Thanks again for all of this great information!


Hi Cary,

Thank you and no worries! I am glad the conversation continues!

Ah ha! I was waiting for someone to fact-check me here!

You are correct, DoF is a physical factor of the lens.

When it comes to DoF math, sensor size sneaks into the equations in the form of enlargement factor, focal length, and subject-lens distance.

I won't dive into the weeds here, because it already too me weeks to hack my way out of the depths of a field of weeds...https://www.bhphotovideo.com/explora/photography/tips-and-solutions/depth-field-part-3

[I wrote that article and it still gives me a headache every time I go back to read it!]

For me, the bottom line is that you can get shallow DoF images with many cameras and lenses, regardless of focal length and sensor size, as long as you employ them properly to do shallow DoF work.

Thanks for stopping by and adding to the discussion, Cary!

Hi there, so to be clear, I understand the distance crop measurement.  What about the aperture?  If a lens has a F2.8 aperture on a crop sensor, should I multiply that by 1.5 as well and the converted/equivalent aperture is now 2.8*1.5 = F4.2?

Hi Pete,

Good question and the answer is, "No!"

There were a few videos online that created some confusion around this topic, but you do NOT adjust aperture for crop factor. If you did, the world of exposure would be turned on its head and many light meters would end up in the garbage!

In general, ISO200 1/400th f/4 is the same on any camera regardless of the size of the sensor. We need these constants so that we can all speak the same language when it comes to exposure.

The only time you would use the crop factor in a mathematical equation that included the aperture is if you were calculating depth of field. And, if you want that equation, I can point you to another of my articles. But, trust me, you don't want it. Just go out and make photos! :)

Explora readers,

I received the following email from a B&H customer and wanted to share their comments and my replies:

Subject: Crop Factor

Message: Hi Todd,
(Have tried to post on the BH forum but have experienced technical issues so hope you don't mind me sending you this query directly)

Thank you for an excellent article. I see you have been discussing the crop factor for some time now and have covered many different aspects in your forum, patiently helping people of all abilities and interests. As a beginner to photography I thought I understood the basic concept of the crop factor, (and based on your article it appears that I do), however I recently had occasion to question my understanding when I sent an ebay seller a query re a lens they were selling. Basically it was a lens built for full frame and its actual focal length was 15-30mm. If I understand the crop factor correctly, to me that would mean that its equivalent focal length when used on an APS-C (Nikon DX) sensor would be 10-20mm (i.e. ÷1.5), however the seller advertises the DX equivalent focal length as 22.5-45 (i.e. ×1.5) and insists that this is correct. I believe that they are either talking about something different than me or they are just wrong. I would be most grateful for your thoughts on this as I have got quite upset over it and have started to question my own understanding. Many Thanks Chris

Hey Chris,

Thanks for taking the time to email after our system didn’t accept your comments.

Thanks also for the kind words about the article!

To answer your question…your eBay seller is correct. You multiply the focal length of a lens by 1.5 for Nikon APS-C sensors to get the focal length numbers that equate to the 35mm-equivlent field of view. So, the 15-30mm lens on your Nikon DX camera would have the equivalent field of view of a 20-45mm lens on a full-frame camera. When used on a full-frame camera, it gives the field of view of a “true” 15-30mm lens.

Also, keep in mind that there are no lenses produced for small(er) sensor cameras that are labeled with their 35mm-equivalent focal lengths. Every lens has a focal length or focal length range (zoom lens) and that number never changes, regardless of what sensor you put behind the lens.

Hi Todd,

Many thanks for your swift and helpful response.

Firstly re the technical problems I experienced with the forum. Basically when I tried to type into the message box the cursor didn't move, so I tried instead to type my submission into a Notepad app and paste it in to the forum message box, but I couldn't do this either. (I would qualify this my saying that I was doing everything on my phone due to being away on holiday and haven't tried it on a PC, so maybe my phone is at fault ?). Hope that helps.

With regard to my question on the crop factor, I have read your response carefully and from what you have said I do believe that my understanding is correct in that you say that :

DX Focal Length × Crop Factor =
FX Focal Length (i.e. 35mm equivalent),

which to me would logically mean that the opposite would apply, i.e.

FX Focal Length ÷ Crop Factor =
DX Focal Length,

wheras the ebay seller insists that :

FX Focal Length × Crop Factor =
DX Focal Length,

which is where I am inclined to disagree with them, as to me the DX focal lengths will always be smaller figures than the FX ones due to the Field of View being smaller when viewed on a cropped sensor.

N.B. If it helps you to decifer the confusion I have, the ebay seller is basing their information on an article they have read on tomsguide.

Having read this article myself I can see why they are insistant that cropped focal lengths are larger than 35mm equivalent ones, but I hope you will agree with me that this is the wrong way round.

Very sorry to labour the point but I need to be absolutely sure that I have the correct perception in my own mind.

If I am wrong then I am happy to be corrected as long as I understand why I am wrong. Hope this is OK.

Yes please feel free to include our conversation thread in the forum as you wish.

Thanks again for your time and patience.

Best Regards

Hey Chris,

Good morning! Absolutely no worries about the response. I am from B&H and I am here to help! :)

I will let our web guys know you are having trouble posting from a mobile device…thanks for the input!

I can see where you are getting confused, and I hope I can clear things up for you.

First off, there is no such thing as a DX focal length. The focal length is a physical distance that does not change regardless of what size sensor sits behind it…or if the lens is even mounted on a camera. A 50mm lens on a DX camera is a 50mm lens on an FX camera is a 50mm lens on a medium format camera and it is a 50mm lens when it is sitting on a bookshelf not attached to a camera.

What changes when you change sensor sizes is the field of view as the smaller sensors will “see" a smaller portion of the image circle created by a given lens. For old(er) photographers, camera companies translated this reduced field of view into a 35mm-equivalent (or full-frame equivalent) “focal length”…so photographers would know if they had a wide, normal, or telephoto lens mounted on their cropped-sensor camera. In reality, the world would be much more simple if we just spoke in field of view numbers instead of trying to translate into focal length designations. But…there is no way I am going to move the needle on that!

So, your first formula is wrong because you are starting with a “DX focal length” when there is actually no such thing.

The formula is:

Focal Length x Crop Factor = Cropped Sensor Equivalent “Focal Length”

Example: 50mm x 1.5 = 75mm

Remember, the lens isn’t actually magically transformed into a 75mm lens, it just produces the same field of view on the cropped sensor that a 75mm lens on a full-frame camera would.

DX “focal lengths” will always be larger than their full-frame equivalents because a larger (longer) focal length equates to a narrower field of view…a smaller section of that focal length lens’ image circle.

How does that explanation fit? :)



More from Chris...

Hi Todd,

Many thanks, I think I am nearly there, please bear with me !

I totally accept that my previous understanding was wrong and so it looks like I will have to eat a particularly large piece of humble pie...

It is your last paragraph which I think is crucial to where I have gone wrong, although I am still just not quite there in understanding why the narrower field of view produced by a cropped sensor results in a larger (longer) cropped focal length. Does it have anything to do with angles ? I think my stumbling block can be summed up as "If the portion of the standard (full frame) lens' image circle is smaller when seen by the cropped sensor, why is the equivalent (cropped) focal length bigger ?" I think you have possibly identified that it is my understanding of Field of View which may be at fault ?

Also, I thought it might help if I explain why understanding this subject has become so important to me. You explained in your article and your last email how people who used to use 35mm film cameras and now use digital cameras with cropped sensors wanted a way to translate the focal lengths they were used to into the equivalent on a cropped sensor. Basically I have the opposite problem in that my first 'proper' camera was a DSLR (Nikon D300) and I got used to all the focal lengths of the lenses made specifically for APS-C sensors. I have now moved to a full frame system (Nikon D750 and FX lenses), hence I wanted be able to do the opposite, i.e. work out the 35mm full frame equivalent of the focal lengths I was used to. I totally accept your point though that DX Focal Length doesn't exist, it was just my way of phrasing the point I was starting from. It seems that my confusion has been added to by sales people telling me that for example my old Sigma 10-20mm (APS-C specific) lens is equivalent to the Sigma (Full Frame) 15-30mm lens. I assume this must be wrong because if I use your formula a 15-30mm lens (designed for full frame) would have an equivalent focal length of 22.5-45mm when placed in front of an APS-C sensor. (Not 10-20mm). Hopefully this illustrates where I am getting my multiplies and divides mixed up.

The other reason I want to understand this properly is that an ebay seller was very rude to me when I asked the question. So much so in fact that they really upset me, so I want to make sure that I'm educated on this so that I don't make the same mistake again.

I very much appreciate your patience and I am sorry for being so slow in picking this up ! (It may be because I am on the Autistic spectrum, and we do tend to have problems with perception.)

Kind Regards


Hi Todd,

I know you haven't yet had chance to respond to my previous email yet but in an effort to do better I have re-read your last email several more times and think I may have worked it out by altering my perspective and using two scenario examples to guide me through the process.

I would be most grateful if you could tell me if I am now on the right track. I am fairly sure that Example A is correct, but Example B is partly based on guesswork, and that's really the important one for me to get right.


Photographer A is used to their 35mm film SLR and finds their 50mm lens ideal for portraits. They want to continue taking their portraits as normal but buy a DSLR with an APS-C sensor. Because they want to achieve the same angle of view of their 50mm lens, but on a cropped sensor, they have to put a larger (by x 1.5) 75mm (full frame) lens in front of the cropped sensor to compensate for the (by ÷ 1.5) cropping which will occur.

EXAMPLE B (Based on me !)

Photographer B has never used a film SLR and consequently has no pre-conceived ideas about focal lengths. The first DSLR they buy has an APS-C sensor and they accordingly buy only lenses made specifically for APS-C sensors. They like taking landscapes and have found that their 10-20mm lens is great for wide-angle shots and they get used to this. They want to continue taking their landscapes as normal but buy a DSLR with a full frame sensor. Because their 10-20mm lens is built to match the APS-C sensor size, no cropping occurs when it is put in front of an APS-C sensor, however if this lens were to be put in front of a full frame sensor, the sensor would still 'see' a 10-20mm focal length, but due to the sensor being larger, this would be confined to the "centre" part of the frame, i.e. it would act (sort of) like a larger full frame lens which has been cropped (with the addition of dead space round the edges), so in order to acquire the same field of view on their full frame sensor as they would have had on their APS-C sensor, they need to multiply the focal length of their original lens (by x 1.5) which compensates for the (by ÷ 1.5) by which this lens has been (effectively) cropped, to give them the desired field of view and hence the full frame equivalent focal length (15-30mm).

I'm holding my breath !

Thanks again

Hey Chris…good morning! I will reply to this email first and then to the other one…

Example A

Incorrect. Remember, the formula is Focal length x Crop Factor = Full-frame equivalent focal length.

The focal length you’ll need for your APS-C x 1.5 = 50 (the focal length you want to simulate)

Answer: 33.333333

Remember, the lens focal length, when placed in front of a smaller sensor, gets multiplied by the crop factor. When the sensor is smaller, the focal length will always be shorter than the full-frame focal length for the same field of view.

Example B

You are correct. You took a while to get there :)….but you are correct.

If you shoot a made-for-DX 10-20mm lens on a DX camera, the full frame field of view you simulate is 15-30mm.
On a full-frame camera, a 15-30mm lens will give the same field of view as a 10-20mm lens on a DX camera.

It gets a bit muddy when we put that DX lens on a FX camera…the lens is still a 10-20mm lens, so you theoretically could have a field of view of 10-20mm on the FX camera…BUT…the DX lens is smaller and is designed to create a image circle that is smaller than the full-frame sensor. So, you will have heavy vignetting (edge darkening) as the image projected is mostly inside the full-frame sensor.

Another way to think about it…if you had a 10-20mm lens that was not designed for DX, you would have a simulated 15-30mm lens on a DX camera and a true 10-20mm lens on an FX camera.

How does that work for you?

On to your first email….

Hey Chris,

No worries at all. It is my pleasure to help you figure this out. This is a benefit to me as well as I constantly learn how to refine my teaching of this subject.

I have eaten a lot of humble pie over the years, myself…don’t worry, it is calorie neutral! :)

I would blame the people who created the strange connection between focal length and field of view, and not yourself. Unfortunately, especially now that sensors are made in different sizes, we are stuck with converting 35mm-film focal lengths to equivalences for smaller sensors.

I am sure you are not the only person who started with APS-C and is now trying to figure out full-frame. The founding fathers of digital photography didn’t give much thought to photographers like you as they assumed everyone was switching from 35mm film!

(See my last email)…Your Sigma DX 10-20mm lens on a full frame camera is a…wait for it…10-20mm lens. But it wont work well on full frame because of the size of the image circle—it is smaller because the lens was designed for DX. If you want to simulate the same field of view that your 10-20mm lens gave you on your D300, you would get a 15-30mm lens for your D750.

Spectrum or not, Chris, you aren’t the only person who is confused by this. And, by the way, your English and writing is exceptional.

Standing by for more questions! :)


From Chris:

Hi Todd,

Many thanks for your kind words and encouragement. You are definitely one of the "good guys" and it is great to have the help of someone who really wants you to succeed, as there are sadly so many people around who take delight in putting you down when you don't understand something.

I am very pleased that with your expert help I have now grasped Example B in terms of the physical process and how this resolves the maths. What a relief ! As I now understand Example B correctly, i.e. moving from an APS-C to a full frame body, I can put that scenario to bed.

Unfortunately I am as confused as ever re Example A - moving from Full Frame to APS-C. This is because (to recap the story so far) :

1. I started out with the belief that APS-C equivalent focal lengths are shorter than full frame ones, e.g. a 50mm lens on full frame is equivalent to a 35mm on APS-C. (Granted this was only what I had read in magazines etc and took it at face value, and it lead me to believe that the formula should be
Full Frame Focal Length ÷ Crop Factor 1.5 = APS-C Equivalent Focal Length

2. Then the ebay seller confused me by saying that APS-C equivalent focal lengths are longer than full frame ones, e.g. a 50mm lens on full frame is equivalent to a 75mm on APS-C. This would make the formula
Full Frame Focal Length × Crop Factor 1.5 = APS-C Equivalent Focal Length
(For the reasons I gave in Example A).
I thought that you supported this view (your email dated 23 Jul 2018, 14:31 seems to do so and even gives the example 50mm × 1.5 = 75mm)

3. However in your email dated 24 Jul 2018, 14:38 you say that the 50mm lens has the APS-C equivalent of 35mm which seems to return me to my original assertion that APS-C equivalent focal lengths are shorter than full frame ones ?

As you can see I have got tied up in knots again and I appreciate that I may not be comparing like with like somewhere along the line. It would help me to know if BOTH the following statements are true :

"To achieve a 50mm full frame-like field of view on an APS-C sensor you would need to attach a 75mm full frame lens'

"A 35mm (APS-C specific) lens placed in front of an APS-C sensor gives the equivalent of a 50mm full frame-like field of view"

If both statements are true, does this mean that they are separate scenarios with different math dependent upon which type of lens ends up against which type of sensor ?

Reading your email dated 24 Jul 2018, 14:38, I would like to check that I am interpreting your formula correctly, i.e. should I read the formula as something like :

Focal Length Wanted (APS-C) × Crop Factor 1.5 = Focal Length (Full Frame) you started with  ?

If this is correct wouldn't it be much easier to find the unknown by doing :

Focal Length (Full Frame) you started with ÷ Crop Factor 1.5 = Focal Length Wanted (APS-C)

Sorry if I am going over old ground..

Please help as it seems that every time I think I have grasped the concept, the opposite answer seems to come my way !

I do of course accept that it is my perception of what you have said which is wrong. To digress I have experienced a similar perception problem when trying to understand the subject of Lens Aperture and its resulting Depth of Field, but I have cracked that one by creating a table of different expressions people use to mean the same thing, which I'm pleased with, as it seems to work really well. I am still curious however as to why a wider aperture has a smaller F/Stop number, but I guess that's possibly a historical anomaly. To remember it I tend to think of it in terms of 'The more of the scene which is in sharp focus, the higher the F/Stop number.' I don't know if this holds up scientifically though ?

Kindest Regards

Hey Chris,

Again, no worries!

Let's go through your bullets…

1.  You are correct here, but you are stating it in a bit of a confusing way. But let’s move on…

2.  The eBay seller is not correct. I think, in your original email, he was correct…but, anyway, in this example he was not.

3.  Zipping back to my 24 July email…my math is correct. On an APS-C camera, a 35mm lens gives you the same field of view as a 50mm lens does on a full-frame camera. Be careful talking about “APS-C equivalent focal lengths”…I think that is where you are gaining some confusion. When we figure this stuff out, we have the actual focal length of a lens and the 35mm (or full-frame)-equivalent focal length.

And your statements:

"To achieve a 50mm full frame-like field of view on an APS-C sensor you would need to attach a 75mm full frame lens”…. No. False. You would use a 35mm focal length lens on an APS-C camera to simulate a 50mm lens on a full-frame camera. 35mm x 1.5 = 50mm.  [All numbers rounded.]. You can work that equation backwards, but why would you as it makes things confusing.

"A 35mm (APS-C specific) lens placed in front of an APS-C sensor gives the equivalent of a 50mm full frame-like field of view”…Yes. True.

One statement is false. The other is true.

Lens Focal Length [doesn’t matter if it is a DX lens or not] x 1.5 crop factor = full-frame equivalent focal length

Two examples:

1.  I have a 200mm lens. When I put it on a DX camera, what will the full-frame-equivalent focal length be?  200 x 1.5 = 300mm.  The 200mm lens will have the same field of view as a 300mm lens on a full-frame camera.

2. I have an APS-C camera and I want to do portraiture with a lens that gives me the same field of view as an 85mm lens does on a full-frame camera. What focal length lens should I get?  Lens Focal Length x 1.5 = 85mm (full-frame equivalent).  Solve the formula and you get a focal length of 56.667mm. Put a 57mm lens on an APS-C camera and it has the same field of view as an 85mm lens on a full-frame camera.

I think that should help, right?

Regarding aperture numbers…the reason they confusingly get smaller while the aperture gets bigger is because the numbers are actually ratios. :). Check out this article: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/understanding-exposure-part-2-aperture

How are we doing now? :). Clear as mud?


Hi Todd,

Great stuff. I think you have identified my misconceptions and filled in the gaps, so using your latest information I have concentrated on the importance of the formula, although as you will see I have not been able to bring the concept together without using the term "APS-C Equivalent" or "Cropped Sensor Equivalent". I'm just not sure how else to express that particular requirement ?

My starting point is to acknowledge that the formula itself remains constant, irrespective of which way around you are doing the focal length conversion. Would it be fair to say then that the formula is more of an equation, with the crop factor rate remaining constant in the middle, and the 2 variable descriptions, i.e. the focal length you've got and the focal length you want appearing at either end as 'qualifiers' to the constant factors, with which way around depending on the type of conversion you are doing ?

If I am correct this would mean that :

Lens' Physical Focal Length (irrespective of which sensor size it was designed for) × Crop Factor = Full Frame Equivalent Focal Length

(Example for converting a 10-20mm lens designed for APS-C Nikon into it's full frame equivalent
10-20mm × 1.5 = 15-30mm)

(Example for converting a 50mm lens designed for Full Frame into it's APS-C Nikon equivalent
??mm × 1.5 = 50mm =
35mm x 1.5 = 50mm
so answer is ?? figure which is 35mm)

How am I doing ?

If the above is correct then the reason I have been confused all along is that I couldn't understand why the formula always said to multiply by the crop factor whichever way round you are doing the conversion. The above is my way of rationalizing this.

Fingers crossed and over to you..

Many Thanks

Good morning, Chris!

I think you’ve got it!

Base equation…correct.

First example…correct.

Second example…almost perfect. It is important to know that, in the world of 35mm-format photography, there is no such thing as a “full-frame lens” or an “FX lens.”This is a source of confusion for many as well. Lenses are either 1) designed for cropped sensor cameras or 2) lenses. So, in your second example, you say “50mm lens designed for Full Frame.” In actuality, you should just say, “50mm lens.”

There are 2 scenarios:

1) You own a 50mm lens and you want to know what the 35mm-equivalent field of view will be, expressed as a focal length, on an APS-C camera.  

Math: 50 x 1.5 = X
Solution: 50 x 1.5 = 75mm

Explained: With a 50mm lens on an APS-C camera, your field of view approximates what you would see with a 75mm lens on a full-frame camera.

2) You own an APS-C camera and you want to know what lens focal length to use to simulate the field of view you remember from when you shot 35mm film or a full-frame camera.

Math: X x 1.5 = 50mm
Solution: 33.3333 x 1.5 = 50mm

Explained: You would use a lens around 35mm on an APS-C camera to simulate the field of view of a 50mm lens on a full-frame camera.

How does that work for you?



Hi Todd,

Success !

You must have read my mind as you have answered the exact questions I was going to ask with points 1) and 2). Thanks also for the clarification re lens description.

Finally, with regard to these 2 scenarios, would I be right in thinking that your 2 points below (taken from previous emails) belong with these 2 statements :

1)  EARLIER E-MAIL - 23 Jul 2018, 14:31

"DX “focal lengths” will always be larger than their full-frame equivalents because a larger (longer) focal length equates to a narrower field of view…a smaller section of that focal length lens’ image circle."

2)  LATER E-MAIL - 24 Jul 2018, 14:38

"Remember, the lens focal length, when placed in front of a smaller sensor, gets multiplied by the crop factor. When the sensor is smaller, the focal length will always be shorter than the full-frame focal length for the same field of view."


It has been a real pleasure conversing with you and I apologise for taking up so much of your time, but I can assure you that it has been of great benefit and encouragement to me, so thanks again. Keep up the good work and if there's ever anything I can do for you (testimonials etc) please do not hesitate to ask.

All the very best

P.S. From reading the whole comments thread on the forum I think you would be in an excellent position to write a whole book about the crop factor, including both your brilliant overall descriptions (including historical context), and also plenty of FAQs and a sprinkle of differing opinions on the accepted scientific facts. I don't know of such a book in existence and it would bring together all your hard work and save you having to re-iterate many of the same points over and again. I'd be the first to buy it !

Congratulations, Chris!

I am glad I was able to help!

Regarding the 2 statements…

1) I probably should have worded that differently as I just confused myself re-reading it! I was not incorrect, but I think I could have written more clearly.

An APS-C specific lens will have shorter focal lengths for a given field of view on a full-frame camera. For instance…the APS-C 35mm is like a full-frame 50 and the APS-C 56mm is like a full-frame 85, etc, etc.

2) I am correct here again…but, again, could have used different words.

Chris, not a problem at all. I love educating people on this stuff and I know crop factor can be totally confusing. A book on Crop Factor? Ha! I don’t think that will be on the New York Times Bestseller list anytime soon, and if anyone ever found my college transcripts and math grades, I would be labeled as a charlatan! B&H is always happy to get positive feedback from our customers via social media, but I would never ask that of anyone.

Take care and let me know if you have any questions about photography in the future!



Hi Todd,

I am in the process of writing up my notes and I think I have finally identified how and why the eBay seller confused me in the first place and I would appreciate it if you could tell me whether I am correct.

The eBay seller was advertising a 15-30mm (standard, i.e. NOT built for cropped sensors) lens

They made the following statement :
“Compatibility: Fits all Nikon FX/Full frame camera bodies. Also works on DX/APS-C/Crop Sensor bodies with an inbuilt focus motor, providing an equivalent focal length of
To me this is extremely misleading in terms of their use of the term “equivalent focal length”.
I think that the Focal Length range they quote belongs to the following scenario :
A 22.5-45mm (standard) lens would need to be placed in front of a DX/APS-C/Cropped Sensor to provide a “true” 15-30mm focal length on a DX camera.
To my mind, most people (me included) wouldn’t be interested in that scenario, but instead would want to know what (cropped) equivalent focal length range this lens would provide if placed on a DX camera, which is actually 10-20mm. (10-20mm x 1.5 = 15-30mm).

Am I right ?


Hey Chris,

The eBay seller’s statement is correct in the sense that it is written how most people discuss “equivalent focal lengths” with cropped sensor cameras. If he wanted to be more correct, he could have used the term “field of view,” but most folks do not use that term when talking about crop factor.

So, I wouldn’t be too harsh on his terminology.

Remember, the formula is:  Lens Focal Length x Crop Factor = The equivalent focal length associated with the smaller field of view.

Any given focal length lens will have a smaller field of view on a crop sensor camera and that smaller field of view correlates with a longer focal length.

So, a 15-30mm lens (regardless if it is a DX or non-DX lens), when it is placed on an APS-C camera, has a field of view similar to what a 22.5-45mm lens would have on a full-frame camera.

In order to get the same full frame field of view of a 10-20mm lens on an APS-C camera, you plug the numbers in to the formula:

X x 1.5 = 10mm
X x 1.5 = 20mm

6.7-13.3mm… Or approximately a 7-13mm lens.

If you want to get the same full-frame field of view as a 15-30mm lens on a cropped sensor camera, you will do the math, same as the above example, and find that you need a 10-20mm lens.

Remember, put any lens in front of a smaller sensor and the field of view will be narrower and that narrower field of view equates to a longer focal length.

Does that clear things up?

Also, don’t buy lenses on eBay :)  Shop at B&H instead!



Hi Todd,

Yippee !  I think its fair to say that I can now do the right math for the right situation and understand the basics correctly. Just a 'loose end' or two really..


There is just one piece of background information which I need to understand in more detail. It is the concept I have struggled with the most, i.e. why a smaller (narrower) Field of View has a longer Focal Length.

I have tried to explain why in the example below, but I suspect this may need some correction/clarification :

"A 50mm lens placed in front of a cropped sensor camera 'simulates' or 'acts like' how a 75mm (standard) lens would behave when placed in front of a Full Frame Sensor.
This is because when the smaller sensor applies the crop, it is a similar effect to zooming in (i.e. using a longer focal length) to reach the portion of the lens' image circle which the sensor can use".

How's this ?


As a beginner I have very much taken on board what you have said about being careful what terminology I use in order to ensure accuracy, so am now trying to use terms which are accurate but also mean something to me. I would contend however that professionals in the photography and sales world should do the same, but for the reason that their terminology can be lacking in detail, and therefore potentially misleading to those like me who don't have the same knowledge and experience. For example, based on the clarification you have just given, both the eBay seller and myself were both correct all along in what we said, it was just that we were both describing different scenarios. I accept what you say though that he was using a comparison which people are familiar with, so I was naive in that respect.

I think the most important lessons I have learned are :

1) Stick to the formula ! (But be aware of whether your 'answer' lies at the beginning or the end of the equation, depending on what kind of conversion you are doing).

2) Use the correct terminology to make the math and science resolve.

Looking forward to your comments.

Many thanks as always

Hi Chris,

I am glad it is all coming clear to you!

Your question: Why is the field of view smaller?

I will give you a metaphorical example to try to illustrate this….

Think of a window in your home. If you stand in a room in a designated spot and look out the window you will see what we will call a full-frame view of what is outside of that window. Now, imagine holding up a smaller window in front of you, surrounded by a frame that blocks out all of the original window. Looking through our new window, we are now only seeing that smaller part of the view. The original window and full-frame view still exists, but we are forced to look at a smaller portion of it.

Now, imagine that window is a camera lens. The round lens gives us a circular “image circle” when we look through it. A portion of that image circle is captured by the sensor. Now, in place of the larger sensor, lets put in a smaller sensor in the same place. The smaller sensor captures a smaller portion of the center of that image circle. The effect of capturing that smaller portion of the image circle is virtually the same as if you had a zoom lens and you zoomed to capture a smaller portion of the frame. The difference is that we are, in effect, cropping the image instead of changing the optics (focal length) to get closer to the subject. This is illustrated in the article in the image showing a circular view of a scenic mountain.

In defense of the eBay seller, he was using the common terminology that most people (and even manufacturers) use. The core of the problem is that we are no longer all (for the most part) living in a standard 35mm-film format world. Photographers, over many years, have translated the angle of view number to a focal length. You can see this number listed on the B&H Photo website under a lens’ specifications. For example, a 50mm lens has an angle of view of approximately 46-degrees. Now that we have smaller-than-35mm film-format sensors, we feel the need to calculate equivalent focal lengths so that those familiar with the familiar angles of view of the 35mm-format lenses can pre-visualize the image a particular lens will give them.

Yes, on the equation, be sure to start with the correct labels and make sure your variable is in the correct spot so that you solve for what you are trying to find.

Again, please let me or B&H know if you have any other photography related questions…or more questions on crop factor.



Hi Todd,

Well I think that ties it up nicely, looks like the end of a mini-era !

I will rather miss our daily 'crop factor catch-up' but I'm sure there are plenty of other photography subjects for me to confuse myself with. This has certainly got my feeble brain working again after many years of inertia.

By the way I was reading in the article about your first camera, and I too had a similar experience in that it was my grandmother who bought me my first camera, which was an Olympus XA3 (happy days !). Yes I was very much around in the film era, but never got around to using an SLR, as my other hobbies of snooker and railways took precedence back then. I have just seen the XA3 in a magazine article about vintage Olympus classics, which does make me feel really old.

Anyway, I think that's it (for now). I am most grateful to you for expertly and patiently guiding me through this minefield, and my wife says thanks too as I have been driving her mad with this for a week and a half, so she should get some peace now.

With very best wishes

Ha! No worries, Chris!

Looking forward to the next mini-era! :)

My best to your family as well.



Nice explanation, but I still see people confused into thinking that by simply applying the crop factor to the focal length, to get the similar FOV means we can expect the same photo - not true of course.  A 50mm lens on my Olympus will NOT give me the same photo (background / foreground relationship) as a 100mm lens on a FF camera even if they have the same FOV.  A 50mm lens is still a 50mm lens. Of course, we don't apply the crop factor to the aperture (or ISO!) either :-).

Rabbit Hole warning, Phillip!

Your life will be much more simple if you don't apply crop factor to aperture or ISO. Trust me! Chris, if you read Phillip's comment, please strike it from the record!

Also, Phillip, you might be surprised at how similar your 50mm Olympus photo looks when compared to a full-frame 100mm for most images. When you have very close and very far objects, you may see a difference, but, honestly, it might not be as different as you might think.

Thanks for stopping by!

You obviously didn't read this carefully.  I said .."we don't apply..".  I have noticed some small differences between my m43 and FF cameras when shooting wide angles - not alot, but the point is, they aren't the same. 

Yes, please delete my comment. I won't bother again.

I have actually had people tell me that a 50mm on m43 is EXACTLY the same as putting a 100mm on a FF!  No, it really isn't - close, but not the same thing.  I think people don't get the difference between FOV and Focal Length when using different size sensors.  I actually think the distinction is not trivial.

Hello again, Phillip,

You are correct...they aren't identical.

Is the distinction trivial? I could probably argue either way. At the core of this issue is that photographers (speaking in generalizations) feel the need to quantify lens field of view as focal length based on one (of many) formats of film. If we all grew up shooting medium format, we would have internet forums debating what focal length was actually "normal" and what was wide and telephoto.

Life was simple for must of us shooting 35mm film. Then digital sensors came out and they were smaller and suddenly the "traditionalists" needed to apply math to focal lengths so they could keep their familiarity with those numbers intact where, for beginners, it really is a bit of a disservice and source of confusion.

We should try...ultra-wide, super-wide, wide, normal, telephoto, super-telephoto, and ultra-telephoto and forget the numbers. After all, Starbucks proved that you can get rid of large, medium, and small by forcing fancy words on coffee drinkers. :)

Thanks for the banter and thanks for stopping by!

Hey Phillip,

Not a bother at all! We enjoy the conversations with our customers!

Hey Phillip,

I actually did read it correctly, but I thought the emoticon at the end of your statement was to indicate sarcasm. A lot of folks have watched a famous YouTube video that says you need to apply crop factor to aperture and ISO as well and I thought you might be referring to that. :)

I was actually referring to that rather unfortunate video - many evidently buy into that nonsense. Last point, one can quibble about the actual impact on your photos of simply applying the crop factor - but like I said - I have actually had people tell me that applying the crop factor somehow magically turns one focal length into another on a smaller sensor camera. So even if you can't see alot of difference - I think it's important to understand the basics.  You tap dance around this issue, but I have yet to see anyone explicitly explain this.  I would love to see someone show this - maybe 30mm on a Nikon 1 vs. an 85mm on a FF camera. On the other hand, I have known a couple of outstanding photographers who appear clueless of even the basics, so... enough said.

Hi Phillip,

I knew it! [...regarding the video] :)

Personally, I recently tested a medium format camera against an APS-C camera and, unless my eyes are going (I used to be an aviator with 20/10 vision), I couldn't tell which image was from which camera aside from their format being different and the increased resolution of the medium format camera.

But, I think it is time for B&H Explora to line up a Micro Four Thirds, APS-C, Full Frame, and medium format (?) camera with lenses of identical equivalent focal lengths and highlight the differences and similarities. Challenge accepted!

Stand by to stand by! :)

Outstanding! Look forward to seeing it. Alot of fuss to make a small point - equivalent doesn't always mean equal. While you're at it, how about comparing the new Nikon P1000, zoomed all the way out to a FF camera with a lens of equal FOV. Ha! 

On a serious note, since the differences are most easily seen at the extremes - may I suggest including at least FF 14mm and 600m. Both are readily available (one used for landscape/astro/interiors and the other for sports/wildlife) and their equivalant for APS-C (9mm & 400mm) and µ4/3 (7mm & 300mm) - not sure what medium format you have in mind, but no problem.  I would also suggest images with compelling foreground and background features. Maybe throw in a FF 100mm for portrait shooters.  

Thanks for your suggestions. Believe it or not, even working at B&H Photo, there are challenges to what gear we can get our hands on for such a comparison, so we need to get a Plan A list together, but be ready to go with Plan B.

I will be conferring with the same coworkers who savagely attacked me and who are, as we speak, making fun of me over at the scuttlebutt, to see what bodies and lenses we can get.

Thanks, Phillip!

Sounds like we have similar co-workers!

FF 14mm Many options, including primes by Rokinon, etc and some zooms by Nikon/Canon

APS-C Sigma 8-16 @ 9mm There are other zooms as well

µ4/3 Olympus 7-14 pro @ 7mm

FF 600mm Sigma or Tamron 150-600 @ 600mm

APS-C as above @ 400mm

µ4/3 as above (w/adapter) @ 300mm

Good luck

Ha! Yep. All jerks here. :)

Your sensor is even smaller than mine, so you must have it worse! Ha!

Good suggestions! Thank you!

Just so you know, a lot of my reputation will be riding on this article. My coworkers are all saving up for medium format cameras while shooting full frame and making fun of me for living in the APS-C world...explaining that my night photography MF vs. APS-C examples are poor and that I would clearly be able to see the difference if I had not challenged the sensors in the dark and, instead, photographed in a studio with professional lighting.

I now loose sleep knowing that I have helped friends and family purchase non large or medium format digital cameras.

Anyway, I digress by recalling fresh personal attacks and ridicule. Just know that my reputation is at stake!

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