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.



HI, Tod:

I know how to calculate the focal length when using an APSC lend on a full frame camera. For example, 24mm lens will be equivalent to 36mm for Sony. But I really need to know about the conversion of the aperture. For example f1.4 will remain f 1.4 or 1.4X1.5=f2.1. Which one is true? you stated the following that is somewhat confusing: 

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

Please help to understand.

Thank you,


Hello Dong,

Thank you for your question!

My statements are true.

Crop factor does NOT affect a lens's focal length and crop factor does NOT affect the lens's aperture.

The definition of f/stop from my aperture article: "The ratio of the opening of a lens aperture when compared to the focal length of the lens—not a measurement, but a ratio—is referred to as an f/number, f/stop, focal ratio, f/ratio, or relative aperture."

f/1.4 is f/1.4...crop factor does NOT apply to f/stops for exposure.

As the f/stop is not affected by crop factor, this allows us to have consistent exposure math and light meters that work for all cameras, regardless of sensor size.

Many get confused by crop factor and aperture because crop factor DOES affect depth of field, but not because it is applied to the lens's aperture.

Please let me know if you have more questions and thanks for reading!




Hi Todd. Can you explain me why you say that the depth of field is affected by the crop factor ?

Hey Todd and company,

You may have already answered this question and if you have I apologize. What i wanted to know is if I am using a 35mm DX Lens for my APS-C DX Nikon d3000, does this change the focal length of the lens? For example, could I assume that my 35mm DX Lens is STILL a 35mm lens because it is on a DX camera body? I understand the idea of crop factor if it were a DX Lens on a Full-Frame camera. I hope that question makes sense and I appreciated reading your article on Crop Factor! Keep up the fantastic work.

Hi Cole,

No worries on the repeat question! :)

If you are using a DX 35mm lens on a DX Nikon camera the focal length of the lens does not change. If you put that lens on a shelf by itself, it is still a 35mm lens. If you glue it to the front of the James Webb Space Telescope, it is still a 35mm lens. :)

The thing that changes, when you change sensor sizes (full-frame, APS-C, etc) behind that lens, is the field of view of the lens. A smaller sensor captures a smaller area of the light projected through the lens and this gives makes it appear that the lens has a longer focal length—it does not.

I hope this clears things up a bit! If not, please let me know.

And, thanks for reading!



Thank you for answering my question Todd and in such a timely manner! You guys at B&H are awesome and I will always come here for the informative articles on Photography. The depth at which you and many others write their articles is quite amazing. Thanks again!


Cole Barlow 

I am using a Nikon D7000 body and my lens is a Sigme 120-300 mmf2.8. I just picked up a sigma 1.4 and 2 converters what willl I be shooting at when I am shooting at 300mm. 
thanks Gordon

Hi Gordon,

Good question.

With the Sigma lens and those two teleconverters, you will get the following at 300mm on the lens:

  • 300mm f/2.8 with 1.4x = 420mm f/4
  • 300mm f/2.8 with 2x = 600mm f/5.6

I hope this helps! Let us know if you have more questions.

Thanks for reading!



May I know if I am using a 100mm lenses on say a 1 2/3 sensor versus a 4/3 sensor, what would be the effective zoom/ range of the lenses on each sensors ? Thanks 

Hi Richard,

Thanks for the question!

Are you referring to the common 1/2.3" sensor? Or a 1 2/3" sensor (which I have never heard of)?

A 100mm lens on a camera with a 1/2.3" sensor has a 35mm-equivalent focal length of 560mm (crop factor 5.6x).

A 100mm lens on a Micro Four Thirds sensor camera has a 35mm-equivalent focal length of 200mm (crop factor of 2x).

And, in case I misread your question, a 100mm lens on a 2/3" sensor camera has a 35mm-equivalent focal length of 390mm (crop factor of 3.9x).

You can find several crop factor calculators with common sensor sizes online and on apps if you need or want to crunch more numbers...or I can do it for you!

Please let me know if I have answered your question and thanks for reading!



Simple question.  If I have a full frame camera that is 24mp, and an APC camera that is "also" 24mp, doesn't this mean that the APC will give more detail than the cropped full frame photo?  In other words, the 600mm lens will give a better image on the APC in this scenario?  Thanks, :-)

Hey Samos,

Great question!

So, there is a bit more to the science, but, theoretically, yes, if you use a 600mm lens on both cameras and crop the full-frame image to match the APS-C image, you should have more resolution in the APS-C image as you are getting all 24MP in the APS-C image and approximately 16MP in the cropped full-frame image.

Will the image be better? That is subjective! :)

Thanks for reading and let me know if you have more questions!



Hi Todd,

If you take the Canon crop factor of 1.6, square it, and then apply it the sensor area difference, you'd get 9.4MP in the cropped full-frame image.  This is the reason I pre-ordered the EOS R7, because cropping my R6 images by 1.6 would net me only 7.8MP.


Hey Peter,

Whoa...whoa. Too much math for my head!

I would check your work, but that is beyond my math skills. :)

Thanks for reading and thanks for shopping at B&H! Enjoy the R7!



Hey Peter I want to know more about this. Can you please share any link regarding it. 

Thank you in advance.

Hi Sangmen,

Let's hope Peter circles back. But, in the meantime, is there anything I can help with?



I am confused about a couple things. My 60D Canon came with a zoom lens that says it is roughly 30-130, because this lens is made for a smaller sensor is the lens focal length adjusted or do I still apply the crop factor if I wish to compare in my brain to my old film camera?

The appearance of having a longer focal length on a crop body camera using a full frame lens is simply because the image is magnified more on the screen or in the viewfinder, right? The full sensor size camera will have all the same information and more so why is it considered an advantage to shoot some things with the smaller sensor when all one need do is zoom in more with the lens or if at max zoom just crop the image after for the exact same shot? I don't know if I am making any sense. 

Hi Steven,

Good question and you are far from the first person to be confused by this!

Your 30-130mm lens (I assume you are referring to a 28-135mm lens) is a 30-130mm lens regardless of what camera it sits in front of...or if it is just sitting on a shelf!

You only apply the crop factor to the lens focal length when using a smaller (or larger) than full-frame (135 format) sensor camera. Therefore, on your 60D, the 30-130mm lens gives you the same field of view as a 48-208mm lens would give you on a full-frame or 35mm film camera.

You will get the true 30-130mm field of view when that lens is used on either a 135 format film camera or full-frame digital camera. The caveat is that, if the lens was designed for smaller-sensor cameras, you might get vignetting when the lens is used on full-frame cameras.

To answer your other questions:


Q: "The appearance of having a longer focal length on a crop body camera using a full frame lens is simply because the image is magnified more on the screen or in the viewfinder, right?"

A: A few things...I hesitate to use the term "full frame lens" because, when we were all shooting 135 format film, there were only "lenses" or "35mm lenses"...not "full frame lenses." It is important to note when a lens is designed for smaller sensors (i.e. APS-C, DX, or EF-S, etc" lenses, but calling a lens a "full frame lens", in my mind, adds to the confusion.

When you mention "appearance," there are actually 2 "appearances" at work here. The first is what the sensor sees and the second is what the eye sees through the viewfinder. When you use any lens on a camera, the sensor only captures a portion of the image circle projected through the lens. How do we know this? Because lenses project circular images and we only capture a rectangular portion of that circle.  A smaller sensor captures a smaller area of a given lens's image circle. This is why the image—the "appearance" of the image—on a smaller sensor looks more telephoto than it would with a full-frame sensor using the same lens.

The second "appearance" is what you see in the viewfinder. Crop sensor cameras have their viewfinders engineered to show 96-100% of what the sensor sees—regardless of if the lens is a crop sensor lens or other kind. A lens created for smaller sensor cameras creates a smaller image circle than a non-crop sensor lens, but the viewfinder of a camera is tailored to fit the sensor (or film) size—not the image circles of different lenses. So, regardless of if you have a full-frame camera or a crop sensor camera or a non-crop sensor lens on a crop sensor camera, what you see regarding the image framing in the viewfinder is more-or-less accurate to what the sensor is capturing.


Q: "The full sensor size camera will have all the same information and more so why is it considered an advantage to shoot some things with the smaller sensor when all one need do is zoom in more with the lens or if at max zoom just crop the image after for the exact same shot? "

A: The full frame sensor captures more of a given lens's image circle. True. Yes, you can simply crop the image to simulate what you might have gotten with a smaller sensor camera, theoretically negating the "telephoto" advantage of the crop sensor. But, the more important, and less discussed, advantage of the crop sensor camera is that the cameras (usually) are smaller than their full-frame counterparts and crop-sensor-specific lenses are smaller as well—a crop sensor 35-150mm f/2.8 lens is can be considerably smaller and lighter than a non-crop sensor 70-200mm f/2.8 lens.


And, you are making sense and, as I said before, you are far from the first person to be confused by crop factor, so don't overthink it.

Standing by for follow-up questions! Thank you for reading!



Let me see if I have this. The term “equivalent focal length” just means cropped field of view? If that’s right, why do we still use the term focal length to describe it? Can we just change our lingo and say field of view? What am I missing?

Hey Bryan,

Good and fair question! And, you aren't missing a thing!

At some point, way before I was involved in photography, it was decided that the best way to describe the field of view of a lens was to use focal length.

I honestly think that using FOV, or even (better?) skipping numbers all together and saying "wide/normal/telephoto," would make things easier, but, regardless of how you describe it, crop factor still would change the numbers.

Thanks for reading and thanks for the comment! Please let me know if you have more questions or wish to work with me to come up with a paradigm-changing system!



Fantastic article - very clear. If I get a DSLR Canon camera body second-hand to use with my existing EF mount lens I have a crop factor of 1.6.
My query is: Is what I will see in the view finder the full frame 35mm view or the cropped view?
In other words, if I see a full frame view I have to adapt that less will be recorded on the sensor. Or is it a case of 'what I see (through the viewfinder) is what I get - despite the crop factor'?


Hey Andrew,

Thank you for the kind words! I am glad you enjoyed the article.

Good question!

You will be happy to know that the answer is simple: what you see is what you get! Your optical viewfinder will show you what the lens sees—more or less (most viewfinders show between 95-100%) of the view. So, don't think about crop factor when you look through the viewfinder.

You actually just uncovered the whole irony about the crop factor "issue." When you look through the viewfinder, you see the framing you are capturing—so why does crop factor even matter outside of purchasing a lens (wide-angle, normal, or telephoto), and/or grabbing the correct lens from your bag for a given scene?

Thanks for reading and thanks for the question!



Thanks heaps Todd - So happy to hear that 'what you see is what you get' through the viewfinder. It is very reassuring. It is very ironic as you say, and really more about being aware your lens will act like a stronger zoom. Though that buggers my 24mm lens which I loved using as it will now work as a 38mm lens. Oh well, can't have it all all, Good news is my telephotos are effectively stronger!
Thanks again
Andrew Quinn

Hi Andrew,

No worries!

Your 24mm isn't dead...it's just different. :)

There are good and bad things about crop factor when it comes to classic lenses. One of my favorites is the Nikon 50mm f/1.2...as much as I wish it was a true 50mm on my FUJIFILM, I do love how awesome it is for portraits as a 75mm. And, if I want it to pretend to be a 50 again, I just take a few steps back from my subject!

So, if you want a "24mm" that plays like your 24mm, you'll need a 15mm. And, while 15mm's for Canon EF aren't super standard walk-around lenses, there are a slew of 14mm options that are great...and all over the price point map!

Let me know if you have more questions and happy APS-C shooting!



Sorry Todd, your great info has brought up another quick question!
We established that regarding looking through a viewfinder with a DSLR and older lenses that "what you see is what you get" - despite the 1.6 crop factor on Canon DSLR with my EF lenses (and possibly adaptor).

Query: Does that also apply for DSLR camera bodies with a mirror as the sensor is blocked? Or do are all digital cameras mirrorless and you only see what the sensor sees?

Thanks again

Hey Andrew,

I love follow-ups and happy to generate deeper thoughts!

On a DSLR, whe the mirror is up, the sensor sees a rectangular portion of the lens's image circle—the circular lenses project a circular image when light shines through them and the rectangular sensor captures light inside of that circle. The mirrorless camera is the same, except there is never a mirror blocking that image circle.

If the image circle is/was smaller than the sensor, you will get darkening around the edges of the frame—vignetting.

Lenses made specifically for crop sensor cameras project a smaller image circle than non-crop sensor lenses. This is why crop sensor lenses are smaller than non-crop sensor optics—because they do not need to project an image circle as large.

I hope that answers that question well. Let me know if you come up with others!



I just switched to fuji and was planning to purchase an x-t3 or t4 for professional use. I did come over from full-frame knowing that the Crop factor in framing would be different. Then i was confused about the f-stop. So the f-stop is actually what the lens claims it is, or do we apply the crop factor also? i suck at math but just want to know, how i need to adapt in my purchases and lighting. 

Hey Rick,

Good question and welcome to the FUJIFILM world!

The f/stop is the f/stop is the f/stop.

While there are some that claim that the f/stop is affected by the crop sensor, the truth is that your handheld light meter (if you have one) does not have settings for cameras with different sensor sizes. An exposure of 1/250th at f/4 is 1/250th at f/4 regardless of the sensor size. If sensor size change exposure math, we would all be in huge trouble!

What does sensor size change when it comes to aperture? Depth of field. Because of the smaller reproduction of the image, the depth of field for a given aperture on a crop sensor camera is longer than it would be on a full-frame sensor. This fact is what sends customers into a panic and is a badge of pride for full-frame shooters. But, the truth of the matter is that you can still get very shallow DOF photos with a crop sensor camera—so don't lose sleep!

Bottom line, don't worry about math in this f/stop scenario. Of course, you probably know the 35mm is a 50mm and the 90mm is a 135mm, but that is easy to remember. Don't sweat the DOF and just take the DOF that your FUJIFILM camera and lens give you and enjoy the shooting experience!

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



Hi Todd. I was really glad to find this article; thanks for putting the time into it. I have a question. I am looking for the exact setup to video a rare bird, at a distance of 50-100 meters. We will be using a Scopecam Lite, which is basically an action cam with a longer lens. The lens is 40mm, but the field of view is only around 10 degrees, and thus a quick-moving bird would not appear (the cams will be head-mounted). I believe the sensor type is 1/2.3. Could you advise as to the equivalency, or crop factor, so that we would get about a 45 degree FOV, but still a magnification? I am thinking same cam with the 16mm lens, but not sure how this would compare to a basic action cam, which does not work well for images at 50-100 meter distance. Thanks, John Williams, Mission Ivorybill.

Hey John,

Thanks for the kind words and thanks for your question!

I did some web searching and checked out the Scopecam Lite manual and the company's website, but I cannot find any information on the sensor size or any detailed optical information at all. :(

Are you asking what focal length scope cam you should use to get a wider FOV? Or are you asking about getting a similar (or wider) FOV with a "regular" camera and lens?

If you want a wider FOV with the Scopecam, you would need to get the shorter focal length version(s).



Thank you again. The sensor size is 1/ 2.9. I am going to try the 16mm lens. Best wishes! John

Hi John,

You are welcome! Sorry I couldn't be more help here. Let me know how it turns out!

Good hunting!



Hi Ken

Very good read and timely for this newbie. My question is also regarding the focal length versus equivalent focal length which as been asked in different ways in the comments and answered by you. I think that I have got the idea but just to confirm this in my language, if I want to get the same image on my Nikon crop sensor camera as a full sensor camera that was shot using a 50mm lens then I can:-

1. Use a 50mm lens and stand further back from the subject or

2. Use a 35mm lens and stand at the same point (roughly given the 2.5mm difference) or

3. Use either lens and stand at a point that frames the shot as I like it. 

Is my reasoning anything like reasonable?

Many thanks


Hi David,

I am not sure if you were directing your question to Ken who posted a few months back, or to me, but I will take a stab at giving you a reply, if that's ok...

Your three scenarios are correct, but the last one kind of hits the nail on the head. It doesn't really matter what the specific number of the focal length of your lens is...you can zoom or move your camera to a position to get the framing and composition you want and then take the photo. The focal length numbers create more confusion than they should, and people could save themselves a lot of headaches by thinking in simple terms of wide, normal, and telephoto!

Your reasoning is reasonable!

Let me know if you have more questions!



Hi Todd

My bad I did intended directing my question to you, thanks for replying.

Thanks for confirming my reasoning. I now need to stop reading articles on this subject and concentrate on deciding which lens to buy.

As I say I am a newbie to photography but my interest is landscape and ultimately nightscape photography in addition to the normal family and holiday shots. My Nikon came with two kit lenses 18-55 and 70-300. I am thinking about buying a wide angle low aperture lens but not sure if I should go for a prime lens or telephoto wide lens. For example some photographers talk about using a prime, say 24mm or 35mm, while others talk about lenses such as 14-28 all of course with low apertures 1.8 to 2.8.

Do you have any thought about this.

Your comments are much appreciated Todd



Hey David,

No worries! :)

Regarding your wide-angle dilemma...part of the problem is that you have a ton of choices!

Starting with Nikon, they don't really have a DX-friendly wide-angle prime lens that is wider than your 18-55mm zoom. They do have a 10-20mm f/4.5-5.6 wide-angle zoom, so that is a safe bet. [https://www.bhphotovideo.com/c/product/1341603-GREY/nikon_20067_af_p_dx_nikkor_10_20mm.html]

But, when I think of wide-angle zooms, the name that most comes to mind is Tokina as they have been making some great wide-angle zoom lenses for DX cameras for years. I would recommend checking out the 11-20mm f/2.8 and 11-16mm f/2.8. [https://www.bhphotovideo.com/c/product/1571175-REG/tokina_atx_i_af120cfn_atx_i_11_20mm_f_2_8_cf.html & https://www.bhphotovideo.com/c/product/1512055-REG/tokina_atx_i_af116cfn_atx_i_11_16mm_f_2_8_cf.html]. The 11-20mm is currently on sale.

And, if you want to go the prime route (always a good choice), Samyang/Rokinon makes some nice wide-angle primes at 12mm and 14mm.

I have owned both a Tokina wide-angle zoom and Rokinon wide-angle prime and found them to be very good options.

Let me know if you have more questions!




Hi Todd

Many thanks for your comments. I will check your suggestions out over the next few days.

Hope you have a happy New Year and stay safe

Kind regards


Hi Todd

Just thought of a quick question.

If I attach a 50mm DX lens to my cropped camera would I get the same image as a 50mm full frame lens on a full frame camera while standing at the same distance from the subject.

I'm sure that this is the last piece of the puzzle for me.


Many thanks


Hey David,

Thanks for the New Years wishes! I hope yours was good and safe as well!

Last piece of the puzzle...

50mm lens + DX vs. 50mm lens + FX ... In order to get the same image framing, you would need to step further away from your subject with the DX camera.

35mm lens + DX vs. 50mm lens + FX ... If you stand at the same place, the image framing will be almost identical.

I hope that finishes the puzzle. But, if not, I am standing by! Thanks again for reading!



Hi Todd

Thanks for your reply. I understand your answer but after re-reading my question I'm not sure that the question was clear enough though, so what I was trying to say is would these two scenarios produce the same image result:

On a tripod mount I mount (in both scenarios the tripod is not moved)

1- my Nikon D5600 cropped camera with a Nikkor DX 50mm lens correctly focused on a subject

2- a full frame camera with a standard (full frame) 50mm lens correctly focused on a subject

According to my understanding of all that I have read is that the answer would be yes. Am I correct?

Many thanks


Hi David,

Sorry. Nope. Not correct.

In that scenario...cameras in identical positions and common subject, you will get two different images. The D5600 will produce a tighter/closer image of the same subject than the full frame camera.

If you put a 35mm lens on the D5600 and a 50mm lens on the full frame in that scenario, you will get basically the same shot.

On the D5600, a 35mm lens will have the same field of view as a 50mm lens on a full-frame camera.

How is the migraine? :)

Standing by for more questions!



Hi Todd,

Don't know why I wanted to get into photography. I have just taken my second Panadol.

So why do they make a DX 50mm lens which, as I understand it is made specifically for cropped camera's, if it gives the same result as putting a full frame 50mm lens on the cropped camera.

Am I starting your migraine yet? :)

My reason for seeking an understanding is I would like to get into astro photography, Milky Way initially, and I'm wondering if/why I could get by with the 18mm kit lens (a DX lens) that came with the camera. Logic being that if my scenario had been correct then I would not need to use the crop factor of 1.5 whereas by using the crop factor my maximum exposure would be 13sec. With a maximum F value of the lens at 18mm of 3.5 I'm not sure if that would let enough light in.

Your thoughts would be very welcome



Hi David,

I hope the Panadol is kicking in! :)

No migraine here...yet! Ha!

So...a DX 50mm lens is a 50mm lens and, on a DX camera, creates the same field of view as a 75mm lens does on a full-frame camera. This may be the crux of your misunderstanding!

A 50mm lens is a 50mm lens when it sits on a shelf, is thrown into the air, or put on any camera—regardless of the sensor size of that camera. 50mm is 50mm.

A DX lens is designed to produce a smaller image circle as the DX sensor is smaller than the FX sensor. This is why DX lenses can be smaller (and lighter) than non-DX lenses.

Let me know if that has created more questions or confusion.

Regarding astrophotography:

You can certainly get by with a kit 18mm lens for astro...its actually pretty good for the Milky Way. Your 18mm lens on a DX camera gives the same field of view as a 27mm lens on a full-frame camera. Pretty wide and good for Milky Way Shots.

I assume that the "maximum exposure" time is the maximum time you can shoot without getting trailing stars. Correct?

And, if that is the question, you've come to the right place...if you like homework reading! :)

I have a 4-part series on basic backyard astro:


Part III has the info on shutter speed...here is the initial paragraph:

Shutter Speed News Flash: The Earth is round and the Earth is rotating—more than 1,000 miles per hour at the equator. This means that we are faced with the option of star trails or star points in our photos. For all but abstracts and star-trail images, we need to freeze the action in the sky overhead. Many photographers use the “Rule of 600” or “Rule of 500” (or now the “Rule of 200”—see the notes below) when figuring out how long the shutter can stay open without the stars showing the Earth’s movement. The formula is: 600/focal length (35mm equivalent) = slowest shutter speed. Example: A 35mm lens on an APS-C (1.5x) sensor has a 50mm equivalent. 600/50mm=12 seconds. So, a shutter speed longer than 12 seconds means the stars will start to streak through the frame as we are moving. To “freeze” the stars, your focal length and the speed of the rotation of the Earth give you a shutter speed limit.

Happy to keep this conversation going...or drop me a question in the astro series, if you'd like!



Hi Todd

This statement put all into perspective 

 "A DX lens is designed to produce a smaller image circle"

Yes you are correct my whole issue was about trying to get more light into the image without needing to get into stacking images in post processing (for now) as well as not creating star trails. Clearly I could do this with either a wider lens or faster lens. The faster lens option was starting to look quite expensive, which I am now coming to understand why thanks to your The Fastest Glass Money Can Buy article which I've just started reading, therefore looking for any option to not have to use the crop factor. Totally given that idea away as well so will put the Panadol away for now.

I will definitely read your series on astrophotography as I know I will enjoy it and learn much from it.

Your offer to keep this conversation going is very much appreciated and I thank you however I'm pretty comfortable that I now understand why the crop factor issue can only be avoided by purchasing a full frame camera, which won't be in the near future for me so will have to use what I have.

I have no doubt though that we will be communicating again, on you Astrophotography blog probably.

Again many thanks and kind regards


Hey David,

I am glad I helped turn on the light bulb here! :)

If you have an APS-C camera, you will always have a crop factor to deal with. It is the nature of the game...and physics/geometry.

A couple of things to leave you with here...

1) Don't lose sleep over having an APS-C camera. They are perfectly good for making photos. All top pros used APS-C in the early days of digital and many still do. I shoot FUJIFILM APS-C and do my commercial work with those cameras. No client has ever looked at my photos and told me that I should shoot full-frame!

2) One of my favorite focal lengths for APS-C Milky Way shots is the 35mm lens. I love the amount of sky you get without getting too wide or too telephoto. And, if you don't have one, the Nikon DX 35mm f/1.8 is an amazing lens and very reasonably priced [https://www.bhphotovideo.com/c/product/606792-USA/Nikon_2183_AF_S_Nikkor_35mm_f_1_8G.html]. It is smaller, lighter, and sharper than a kit zoom. And, at f/1.8, it lets a lot of light in! More homework: https://www.bhphotovideo.com/explora/photography/buying-guide/the-one-lens-every-photographer-should-have-and-use-the-50mm

Standing by for more correspondence!

Have a great week!




Questions: 1. On your file strip measurement you are measuring the vertical and including the sprocket holes (which shows a measurement btw of 34mm) but I thought that the measure should be the width of the frame itself.

2. your compute the crop factor by computing the diagonal but I have seen the simple calculation by width of the the frame ex.  36/24 (full frame divided by APS-C) = 1.53  Can you comment on which is correct or most accurate?  Thanks!!!

Hey Ken,

Yikes! All these years later and you are the first to notice the error in the graphic! I will see if I can get that changed.

Regarding the diagonal vs. width calculation, I honestly don't know if one is more accurate than the other. APS-C sensors usually are shown to have a 1.5x or 1.6x crop factor and I have never seen it carried to the hundredth decimal place on any literature.

My follow-up, out of curiosity, would be to ask why you are looking for a more accurate crop factor?

Thanks for reading!



Regarding Ken's question about calculating the exact crop factor: The difference in width would give us the crop factor for the width, and the difference in height would give us the crop factor for the height. Since a 'field' is two-dimensional and not linear, it makes more sense to use the diagonal line that varies depending on the length AND height. However, if the same ratio between length and height is used for both full-frame and smaller sensors, namely 3:2, then the relative difference of width (or height) alone will amount to exactly the same as the diagonal relative difference. Therefore, Ken's idea is equally exact, just quicker to calculate since we don't need to calculate first the diagonal lengths in both sensors. Does that make sense?

Hi Dietrich,

I think that makes sense! :)

We are running the risk it introducing too much math to the process of releasing a shutter. And, if you saw my high school and college transcripts, you'd see that my hair should be hurting right now.

Thanks for reading!



Yes, let's shoot! I wanted to thank you so much for your explanations in this post. I am glad that I found it because the crop factor was on top inside my mental bag of question marks. Now, that bag is lighter!

Hi Dietrich,

You are very welcome for the explanations!

Time to drop that mental bag and go shooting, but let me know if more questions surface!

Thanks for reading!



In relation to Ken's question, is your diagram with the measurements of the 35mm filmstrip really incorrect?

I have come across various other diagrams measuring the filmstrip edge to edge and that includes the sprockets perforations within that measurement. 

Hi Chris,

It actually does look correct. I just pulled a roll of film out of the fridge and broke out a metric ruler!

I should have checked this earlier! Thanks for the note and thanks for reading!



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