Photography / Tips and Solutions

Understanding Crop Factor

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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.

 

166 Comments

I think I understand the crop factor in term of the photo, my question is around the description of lens.  Given that the Canon EF lens can be used on both the 5D (full frame sensor), and the 7D (APS-C sensor) is the focal length of the lens always given in terms of the full frame sensor?  Put another way if I am using a Canon EF 70-200mm lens on a 7D am I getting 70-200mm field of view or am I getting a 112-320mm field of view? And conversely if using it on a 5D am I getting 70-200mm or 44-125 field of view.  

Assuming the answer is lens size is given in terms of a full frame sensor (for all lens makers)?  

I still don't understand one thing. If it's a crop factor it means that the image will be cut on the edge correct? So let's say I use 2 20mp nikon cameras, one dx and one FX, both fitted with a 300mm lens to take some pictures at birds. Will the center of the image where I place the bird be the same or will the dx magnification be as if I was using a 450mm? I mean just the center of the image I understand that the field of view is different but I am interested in the magnification of the image. I hope you can understand what I am asking. 

Hello Noberto,

Great question. Let's see if I can clear this up for you.

Two Cameras—one DX, one FX. Both 20MP. Two 300mm lenses. One bird. Two prints of the same size, or same size computer screen.

The bird in the photo from the DX camera will appear to be closer to the camera—the bird will be larger in the viewfinder and in the image/print. This is because the DX camera's image will be a virtual crop of the FX camera's image.

So, theoretically, you could crop the FX image to an identical field of view of the DX image. The difference is that the DX image has 20MP of data, and the cropped FX image would have 13.33 MP of information in the same shot.

Does that clear things up? If not, let me know!

Well yes I undestand that since the 2 cameras are 20mp in the croped area of the FX image I have more pixels in the DX. BUT BUT BUT... the problem stands if we take the megapixels out of the picture and consider the plain image, as the one we can see throught the viewfinder. Now the full frame just has a larger field of view ... correct? So the center HAS to be the same. Even if I can be fooled by the fact that with the larger field of view the center (in this case the bird) looks smaller, it has to be the same size as the bird in the DX camera, otherwise I would be changing the magnification of the lens. Now let's not forget that the focal lenght of the lens can't change in any possible way unless we had a different focal plane. So the magnification of the center of the image has to be the same. I understand compleately that the problem exists ONLY because we count the mega pixels in the picture, but taking that out of the equation, am I right or not?

Norberto M. wrote:

Well yes I undestand that since the 2 cameras are 20mp in the croped area of the FX image I have more pixels in the DX.

I ment the oposite. 

Hey Norberto,

Yes, you are correct. To the camera, the image projected by the lens is identical in size regardless of the size of the sensor—the DX camera just shows you a cropped view of the same image. You are not increasing optical magnification with a DX camera. Lens magnification and field of view are independent of the size of the sensor or film.

Good question!

Hi Todd,

I am mostly generally ok with understanding crop factor interms of sensor size and it's impact upon field of view etc etc. I came by this article whilst searching for a definitive answer on the subject of why crop factor affects shutter speed e.g. in the case of the 500 rule and 1/focal length. 

You yourself, in this article's comments have answered the question posed so often with a simple yes you must take it into consideration. What I'd like to understand is exactly why it needs to be considered. I have attempted to think this through logically but cannot rationalise several 'seemingly' conflicting things. Let me explain:

When I consider the impact of crop factor on the shutter speed required to reduce camera blur my confusion is partly resolved. This is because, as I see it, camera blur is a function of the movement of the subject across the sensor during the exposure. Since the focal length of the lens has not been changed from full frame to crop, the same amount of lateral camera movement must result in the same movement across the sensor. Now, I must consider the pixel density of the sensors in question. If both sensors have the same pixel count, the density is of course greater on the crop sensor and this does mean that for the same lateral movement the subject will move across 1.6 (Canon in my case) more pixels compared to the full frame body. It therefore makes sense to me that 1.6x faster shutter on the crop would balance this effect. But I need to clarify that this is not about the lens magnifying the image on the sensor by 1.6x. Now come the problem(s). If I was shooting with the crop sensor at 1.6x the distance to the subject compared to the full frame, thus producing the same subject size at 100% crop, would this not have exactly the same effect as having 1.6x the shutter speed in terms of the physical movement of the subject in relation to the pixels on the sensor. Conversly, If I were to shoot at the same distance then crop the full frame and enlarge 1.6 to get the same subject size what would happen then?

The thing I'm really struggling with though is understanding the need to factor crop into the 500 rule for avoiding star trails. I can just about understand the need to adjust the shutter speed as the focal length increases, if I assume this is because the longer focal length lens will always result in a larger image on the sensor at infinity focus. Here's the thing though. With star trails my camera is fixed on a tripod and the trails are a function of the stars movement in the sky. The stars speed through the sky is fixed in terms of degrees or radians per unit time about the pole star (I recognise that statement is not entirely 100% accurate but the argument is not affected). Here's the cause of confusion for me. Whilst the stars speed of rotation is constant, the distance travelled in a given field of view is not. So if I were to point my lens centered on the pole star on a crop body I see less stars than on a full frame with the same lens, furthermore, the additional stars in the full frame field of view are are necessarily covering more distance on the sensor. The inevitable conclusion of this is a contradiction that the full frame sensor needs a shorter shutter speed to avoid trails in the extended field of view that was not captured by the crop sensor.

I really hope you can pull all of this apart and explain to me and maybe others why I've got this all wrong.

Thanks in anticipation...

Hey Shaun,

Superb question. Well-written, too!

The manner in which you arrived at your query would lead me to believe that the 500/600 rule applies to both full frame and crop sensor cameras. I do a lot of astrophotography and have used the 600 rule successfully with APS-C cameras, but I haven't really done a super scientific test. And, as you mentioned, the rate of the star's varies based on your position on the planet and their position in the sky...so it won't ever be an exact science for the casual shooter. The math is accessible, but who wants to crunch those numbers in the dark?

I can see how people would just use the 35mm-equivalent field of view in their calculations, or they could just change the formula to make the math work for them. But, as you stated, the motion across the sensor is identical with the same lens on a full-frame vs. crop sensor camera. If you changed focal length, the new field of view/image circle would produce different pixel distances across the frame per a given shutter speed.

B&H night photo guru, Gabriel Biderman, wrote this article a while back and uses different "rules" for the different sensors: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/epic-battle-between-choosing-star-trails-over-star-points

I hope that didn't read like I danced around the answer. My advice to you: Use your own gear and do some tests. Peep some pixels! The popular app, Photo Pills, does crunch some numbers for you and shows both the 600 and 500 rule based on minimal declination of the stars in the frame. And, realize that even though it could be an exact science, these "rules" are more guidelines.

Standing by for follow-ups!

Thanks for taking the time Todd,

On first read of your answer I did feel a bit cheated, but your advice is sound. Who needs to understand the complicated maths (I'm a Brit), when it's the results that count? Much better to use the guidelines as an entry to understanding your own gear and develop a flexible and experimental approach in the field rather than break out the rule book for every new shooting challenge. I shall try and temper my need to know and shoot more instead !!!

Cheers

Hey Shaun,

Sorry you felt temporarily cheated!

As tempting as it is to start crunching numbers, my math skills tell me that I am not the guy to do it. Rest easy, however! Others have! Use PhotoPills or check out some other more techy blogs if you crave some equations! :)

But, yes...the proof, as they say, is in the print (or on the screen).

Here is a paraphrased anecdote for you: When the Apollo Lunar Module (LM) was being designed, one of the astronauts engaged in the engineering process was Pete Conrad—a former Naval Aviator. The designers showed him a pressure gauge for some sort of oxygen tank. They said, "When the tank drops below a certain pressure you will see it on the gauge and this light will come on and you will be in trouble."

He asked, "What do I do when the pressure drops and the light comes on?"

"There is nothing you can do about it," the engineers said.

"Well, I'll tell you what. Why don't you get rid of both the gauge and the light, because if I am going to die, I want to die calm."

I used that same argument (and failed) when I was told I needed to memorize the temperature at which the tail rotor gearbox hot light came on in the H-3 Sea King (140F + or - 2 deg). There was no gauge to show the temperature, so why did it matter what the temperature was?!

Knowledge can be dangerous. :)  Go out and shoot using the 500 or 600 rule and see what you get! Also, check out this LonleySpeck link: https://www.lonelyspeck.com/milky-way-exposure-calculator/

Cheers!

Thank you for a great article, I'm only mildly confused on one last thing. Does crop factor in regards to field of view only apply when using lenses that weren't made with that sensor in mind?

For example, 50mm ef mount lens made for full frame sensors on a micro 4/3 sensor will have a field of view of 120~

But, will a 50mm m43 lens on an m43 sensor also have the crop factor applied, or will it have the fov of a 50?

In even simpler terms! Veydra 13mm m43 mini prime on a black magic pocket cinema camera, results in a 13mm fov or a 30mm~ fov?

Hey Vin,

Great question.

Any 50mm lens built for, or adapted, to a Micro Four Thirds sensor will have a field of view equivalent to that of a 100mm lens on a full-frame camera.

And, the Veydra 13mm on a Micro Four Thirds camera will have a field of view equivalent to that of a 26mm lens on a full-frame camera.

This is why the "Nifty 50" options for Micro Four Thirds are 25mm lenses. From the outside, it looks confusing, but if the fields of view changed arbitrarily, we would have quite a mess of mathematics on our hands!...especially for those adapting different lenses to smaller sensors.

Thanks for your question!

Hi Todd,

Thanks for the thorough explanation.

The thing that I don't get now actually has nothing to do with the technical side of this. I just don't understand why almost every photo online with the listed metadata next to it (like Flickr does) doesn't list the 'equivalent focal length' instead of the normal focal length, or at least mentions the crop factor.

You're supposed to look up the camera in order to find out if it's a full frame or not and then calculate the equivalent focal length. Or am I just wrong in thinking they display the regular focal length?

Thanks again.

Hey Vince,

You are welcome.

Unfortunately, I think you need to do the math in your head as Flickr just mentions the camera and lens type—not the 35mm-equivalent.

Yep...just checked my own Flickr and it only shows the focal length of the lens, not the equivalent focal length on a cropped sensor camera.

Fun fun!

Thanks for reading!

Do you consider crop factor when finding the minimum hand held shutter speed? Using a 50mm 1.8 on a crop sensor. Would it be 1/60 with no crop factor figured, or 1/125?

Hey Steve,

Great question!

When figuring out theoretical hand-held minimum shutter speed, I would definitely use the 35mm-equivalent focal length...so, 75mm in your case if you are shooting 1.5x APS-C.

Usually the rule is: 1/focal length. So, on a full frame, you need to take care shooting slower than 1/50th of a second. On APS-C with the same lens, 1/75th.

Shoot steady!

Hi todd

i thought I had this, but after rereading again, I may have confused myself

i have a Nikon  APS-C crop body and shopping for lenses   I bought a FX 70-200 lens, which I understand from your explanation will give me an 35mm equivalent of 105-300 at the 1.5 CF.  So far, so good.   

What is getting me now is the difference between buy a FX lens and a DX specific lens.  Originally, I had thought that the CF applied only to the FX lens when used on a APS-C body, and that design of the DX lens meant that the "focal length" as stated did not need the CF "adjustment".   Thus, for example, an FX 24-80 lens on my camera would yield the 35mm equivalent of 36-120mm, but if it was a DX 24-80,  it would give the 35mm equivalent as "stated", just 24-80, no CF adjustment needed as it was a DX lens on a APS-C body

ok - that is what I thought, but now thinking the CF applies to both lens.  The only difference between the two lenses being that the image circle in the DX lens is smaller and thus will not work well on an FX body

so, is my understanding right now, or was it correct the first time?    I think it is right now - the CF applies to FX AND DX lenses.   Why is this important.    I am trying to by 1-2 shorter lenses and want to make sure I have he right "overlap" with a shorter zoom I am considering 

thsnks in advance

Hi Todd. 

Any thouhts as to whether I had it right the first time or now (now being that CF applies to any lens used on a APS-C body, regardless of if it is a FX or DX lens?

Hey david,

See below. Thanks and, again, sorry I missed your question! I got inundated with questions and comments last weekend leading up to the eclipse and yours, unfortunately, fell through the cracks as I was answering them while on vacation and away from our "system."

Hey david,

Sorry I missed your post! I apologize!

Good question...and you aren't the only one confused by this.

First off, there are only DX lenses and lenses that are not DX. There is no such thing as an FX lens.

Secondly, the lens's focal length is a physical measurement and not related to the size of sensor that the lens is placed in front of. That is why lenses on point and shoot cameras have incredibly short focal lengths, but give "normal" perspectives when placed in front of tiny sensors—for instance, the lens on an iPhone has a focal length of around 4mm. Pretty wide on a "normal camera!"

So a non-DX 24-80mm lens on a DX camera gives a 35mm-equivalent field of view as a 36-120mm lens on a full-frame or 135-format film camera. A DX 24-80mm lens on a DX camera gives the exact same 35mm-equivalent field of view as the previous lens. The difference is that if you put that DX lens on an FX camera, you would get some vignetting as the image circle projected is smaller than the non-DX lens.

So, yes, crop factor applies to both lenses in your example. In fact, it applies to any lens you put in front of that sensor.

Again, sorry for the delay in replying!

Thanks.   That is what i got the second time I read it.    Once I started thinking about it, it meant I needed to have one end of my "walking around zoom" very short and the other end medium focal lengt, so my "16-80" lens gives me the equivalent of a 24-120 on my 1.5CF body- will be good and an not give me too much  overlap with the longer lens that I have.    Thanks so much

Hey david,

No worries! That 16-80mm lens is very very good. https://www.bhphotovideo.com/explora/photography/hands-review/nikon-af-s-dx-nikkor-16-80mm-f28-4e-ed-vr-lens

It is a perfect everyday workhorse lens.

Thanks for reading!

Hey Todd, 

Finally the answer i was looking for! As a newbie in photography, i was very confused in whether the crop factor was applied to any lens out there. I bought a DX 35mm f/1.8 for my crop sensor camera, so that means that it's equivallent focal length is 50mm for a full frame dslr, right?

Thanks again!

Hey Matthew,

Kind of right...let me try to rephrase it to be clear:

On your DX camera, your 35mm f/1.8 lens will give you the same field of view that a 50mm lens would give you on a full frame camera.

Crystal clear?

Thanks for reading!

Any ideas of the crop factor of a Nikon P900 with 83mm - 2000mm optical and digital zoom please. And how to work it out as am very confused by this. Trying to take an on-line photography course and struggling a little bit. Thanks for your help. Regards. 

Hey Iain,

The crop factor of the P900 is 5.62x. Pretty extreme to get that much telephoto action!

Also, don't worry about digital zoom, that really isn't a function of crop factor. Digital zoom is simply zooming on on the optically zoomed image.

Check out this link for some help with your course (hopefully): https://www.bhphotovideo.com/explora/photography/tips-and-solutions/introduction-learn-photography

Good luck!

Thank you Todd. Much appreciated 

iain. 

No worries, Iain! Cheers!

Hello!

I have been researching for hours and have been reading so many articles and I still cannot comprehend this:

I use a Canon T2i (crop sensor, 1.6x crop factor), and a 50mm 1.8 EF lens.

I understand why a focal length of a 100mm on an ef lens on a crop sensor would technically make it 160mm...

So why isn't my 50mm technically an 80mm? Why in this case does a 50mm stay a 50mm, and doesnt get an increased focal length? Is it because it's a prime lens?

Hey Karlie,

You are not alone in your confusion.

The focal length of a lens is a fixed measurement. If you take a 100mm lens and put it on a bookshelf, it is a 100mm lens. If you attach it to a large format camera, it is a 100mm lens. If you tape it on the front of a disposable point-and-shoot camera, it is still a 100mm lens. Prime lens or zoom...same thing. The focal length is a measurement.

The size of the sensor of the camera changes the field of view of the lens—not the focal length. We (well, not I) invented the 35mm-equivalent focal length moniker to translate the lenses into a system where most people were familiar—that of the 135 (35mm film) format.

More confused now? Basically, when we talk about crop factor, we translate focal lengths into a "language" that pretty much only those who shot 35mm film understand. I think I am digressing...

I hope that clears things up and I didn't tell you too much!

Let me know!

Thank you for replying, so let me see if I got this right now.

So, same as before, my 50mm on my T2i gives a field of view of 80mm, yet it is still a 50mm lens? Is that what you are trying to say? From the article you said: "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." So in this case, that is the 80mm that I'm trying to figure out, yet it's on my 50mm lens.

So when I say, my 50mm on my t2i is technically 80mm, how is that still wrong? Because it's only the field of view in the lens that makes it 80mm?

So the 80mm is the "equivalent focal length of the new field of view afforded by the smaller APS-C sensor" since "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." Except mines a 1.6x, so hence the 80mm.

Did I get it right this time?

Also, to clarify, does the 1.6x mean that my T2i sensor is 1.6x smaller than the full frame; hence giving me the field of view of an 80mm, yet its with my 50mm. Is that correct? 

Hey Karlie,

You are close to a breakthrough! :)

All of what you said, is correct. Except, you cant really call a 50mm lens an 80mm lens. You cannot call a honeycrisp apple a red delicious apple. Both are apples, but they are very different. Right?

You can say that your "50mm lens has a 35mm-equivalent field of view of an 80mm lens." That is still confusing, right?

Here is the missing part that I did not mention in my reply yesterday. Field of view is labeled in degrees, not in linear millimeters. So, when we talk about 35mm-equivalent focal lengths based on changes to field of view, we are translating degrees of measurement into a focal length that provides that field of view when used on a 135 format (or 35mm format or full-frame) camera.

So, here are your questions, one by one:

  • So, same as before, my 50mm on my T2i gives a field of view of 80mm, yet it is still a 50mm lens?
    • Yes, a 50mm lens will always be a 50mm lens regardless of what you do with it.
  • Is that what you are trying to say?
    • Yep! :)
  • So when I say, my 50mm on my t2i is technically 80mm, how is that still wrong?
    • It is not technically an 80mm lens. It simply provides the same field of view on your camera as an 80mm lens would on a full-frame camera. It is still a 50mm lens.
  • Because it's only the field of view in the lens that makes it 80mm?
    • The field of view makes it the equivalent of an 80mm lens on a full-frame camera, but the focal length of the lens is still 50mm.
  • Did I get it right this time?
    • Yep. Mostly. B+ :)
  • Also, to clarify, does the 1.6x mean that my T2i sensor is 1.6x smaller than the full frame; hence giving me the field of view of an 80mm, yet its with my 50mm. Is that correct? 
    • Yes. Exactly.

Here is the other thing to keep in mind. Just because we use focal length to describe the field of view of camera doesn't mean it is important to photography, in a sense. The field of view of the camera is what you see when you look through the lens—the number assigned to it doesn't really matter. We could use "wide-angle," "normal," and "telephoto" instead of numbers, but the numbers allow us to be more precise when describing the angular field of view (by using linear measurements...ugh). :)

Get it? Got it! Good! ....right? Let me know if this is clear as mud now. 

Thank you Todd for answering all my questions! So in order for my 50mm to have a field of view of 50mm, looks like I need to get a full frame?? ;)

No worries, Karlie!

Yes, if you want your 50mm to have a 50mm field of view, you will have to shoot full-frame.

If you want the 50mm field of view on your APS-C camera, you need to get a lens with around a 35mm focal lenght.

Check out this article! https://www.bhphotovideo.com/explora/photography/buying-guide/one-lens-every-photographer-should-have-and-use-50mm

Good chat!

Hi Todd! I currently have a Canon T3i and I am going backpacking through southeast Asia in a few months and I am thinking about upgrading, I am just starting out with photography and am trying to learn myself. I'm getting to the point to where I need some new gear so I've been shopping around before the trip. I received my camera as a gift and for some reason, I have been unhappy with it lately. I've been researching different cameras and I'm having trouble deciding what to get, especially between a full frame vs. an apsc. After reading I sort of understand (I think) but I still am not sure which would be best for what I need it for, I was leaning toward full-frame but now I'm not sure. I shoot a little bit of everything so it's been hard to narrow it down. I would love a recommendation for a camera if you have one or at least a little help deciding between full-frame or aspc. (If you want to throw in a couple lens recommendations too that would be great)

Hey Lauren!

I am here to help, but your question is more complicated than you might think. I could recommend some cameras and lenses, but I would like to know more about what you photograph and why you are unhappy with the T3i and what lenses you have currently. May you share some more info?

Also, some homework for you: https://www.bhphotovideo.com/explora/photography/features/dx-vs-fx-its-not-debate-its-choice

And, if you want to skip to the end, I would say you will find the Canon T6i or T6s a nice upgrade for your T3i. Before we dive into full-frame versus APS-C, we need to chat more.

Standing by for follow-ups!

Thank you! Also, I'm sorry I meant a Canon Rebel T4i not a 3i. I  Mostly I like to photograph nature, landscapes but also close ups of flowers or animals. I also travel a lot so most of my photos come from my trips. I have a lot of architectural photos, portraits, and landscapes. Really just a little of everything. I suppose I have just been disappointed with the quality of some of my photos, I feel like the images aren't as clear or bright as they could be and I felt like they were always cropped in farther than I remember when I took them. I especially noticed this when I started shooting portraits. I would be careful to make sure to crop at a certain point to not cut off finger tips or toes or something in a portrait and then when I went to edit the photos the toes or whatever I was trying to keep in the image would be much closer to the edge or even touching the edge of the frame. I started trying to shoot from farther out but it kept happening. When I started reading about crop sensors I thought that might be the problem, but then I read a little more and I wasn't so sure. I might also be unhappy with my camera because I started with traditional film and solely used my film Nikon for a couple years until I moved out of my home town and didn't have access to a darkroom anymore. Then I learned digital on my moms Nikon, I forget what she has but I think it is a bit nicer than mine. It took me a long time to adjust to digital, and I must admit I am a little resistant to technology so I thought I was just unhappy with my photos because I didn't quite know how to use my camera. I felt like I could never get the settings quite right and the images always came out just a little dull or not as sharp as I would like. I read all the tutorials I could find and kept trying to play with settings and figure out the problems and it got a little better but then I went on a camping trip to Colorado and my uncle lent me his Canon EOS 70D and I loved it. I'm not sure why but I never had trouble with the settings, everything seemed so much simpler to use and more natural and the images turned out bright and crisp, I barely had to edit anything, I didn't notice much on the cropping but I was shooting more for fun so I wasn't paying as close attention. So I was thinking I was going to try and get one of those but then I started looking up more articles and reviews and got very confused. As far as lenses I have 50mm 1.8 and then the lenses that came with the camera. I know I want to get a few more lenses and I think I picked out a few but I really don't know if it is worth upgrading the body. Thank you again for your help!

Hey Lauren,

Thanks for all of that. It is very helpful.

Here are a couple of thoughts and suggestions...

"Closer crop." First of all, you are doing everything right by paying attention to the edges of your frame and composing with care...so keep it up! What I think you are discovering is the fact that the T4i's viewfinder is a 95% viewfinder. It doesn't show everything the lens sees. What you see is not exactly what you get. So, start helping yourself by leaving a bit more room around the edges of your shot...5% worth!

Canon vs. Nikon. Go with what you are comfortable with. Having come from Nikon, those cameras should be easy for you, but you expressed that your uncle's Canon and your T4i settings are working for you, so you can flip a coin if you want.

When you mention the "lenses that came with the camera," I assume you mean something like an 18-55mm and 55-200mm kit lens, correct? Do you use your 50mm f/1.8 much? My guess is that you are getting better sharpness and color with that lens than the kit lenses. If it is sitting on your shelf, take it out for a spin!

Why do I mention this? My guess is that your kit lenses, not the camera, are holding you back. The T4i is a bit long in the tooth, so it might be time to upgrade, but my guess is that if you stayed in the APS-C world and upgraded your optics, you would be just as happy as if you went full-frame and upgraded your lenses as well. After all, your uncle's 70D is APS-C as well. What lenses were on his camera?

So...the 70D is an option as is the T6....for an APS-C upgrade. The 7D is a great camera as well.

If you want to jump to full frame, the 5D is going to be your go-to option...and you could probably save a good bit of money by getting the superb Mark III instead of the Mark IV.

For upgrading lenses, I would start looking at Canon L-series glass. Expensive, but lenses are a long-lasting investment.

Standing by for follow-ups!

[Continued from previous post]

Also, does crop factor act the same while filming as compared to still photography? Thanks.

-Nick

Hey Nicholas,

Good questions!

To answer your first: No, the focal length of the lens is its physical focal length. Manufacturers will not describe a lens by its full-frame equivalent focal length. So, a 50mm lens is a 50mm lens regardless of what kind of camera you mount it on.

And, when shooting video, the crop factors are also factored in, but, in video, there are many different formats, so, for example, if you are shooting Super 35 like Sissy in the conversation below, there is a different crop factor.

Thanks for stopping by!

Hi Todd!

I am about to buy a Sony A6300 and am a film student so I am just trying to understand cameras down to every detail. Crop factor is a little confusing to me (go figure). My question is, when I go to buy my Sony E-Mount lenses will they have the crop factor already factored into the listed focal length. For example, when I buy a 50 mm prime lense I am truly getting a 50 mm prime lense for the APS-C censor? Or do I still have to keep in mind the 1.5x crop factor when I try to get an exact look/exact focal length? Very beautifully written article though, thank you!

-Nick

Thanks for the kind words, Nicholas! See my reply above!

I don't understand you language " 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."

What is the significance of light rays exiting the lens? 

Shouldn't your sentence just be " Therefore, a 50mm lens can measure 50mm from the point where light rays enter the lens until they arrive on the image plane."

Hey Richard,

Fair point, but light rays have to exit the lens in order to get to the image plane. As far as I know, there is always a gap between the back of the lens and the film or sensor. That gap is often filled by the shutter mechanism and, in the case of a SLR, a mirror.

Maybe I could have been less wordy!

Thanks for reading!

Hey Todd,

I am curious, if I am standing at point A shooting with a crop sensor and 85mm lens to point B, also standing at point A shooting with a full frame sensor and a 135mm lens, I should have roughly the same framing. My question is, is the crop sensor a lower quality image and how does it differ from say zooming in post on the same A to B shot with a 85mm on a full frame? Hope that makes sense! 

Hey Brandon,

Great question, and it does make sense...assuming a Canon APS-C 1.6x crop factor.

The answer is a bit complicated, but, in general, there will not be a difference in image quality and, yes, the framing will be nearly identical.

On the full-frame image, the background may be a bit less in focus for a given aperture as the focal length is increased and depth of field changes.

If both cameras are 20MP, for the sake of argument, the full-frame will show a bit less resolution on the crop as you are discarding pixels while cropping. The cropped area will be approximately 12.5MP where as, on the APS-C camera, you will still get the full 20MP. Does this make a huge difference? No. I would say not.

One last comment. This is a pretty good thought experiment, and many of us have pondered this when thinking of the advantages/disadvantages of certain sensor sizes, however, it isn't usually a real-world exercise, right? Framing and composition are determined by the focal length of your lens and what you see in the viewfinder—not the sensor size of your camera.

Good question!

Hi Todd,

Thanks for this, I was wondering if you would be able to help me with this. If I adapt lets say a Mamiya sekor C 645 lens to EF mount for a super 35mm video camera (Black magic Mini U Pro for instace), what would the crop factor/equivalent be there? Im just having trouble getting the conversion from medium format to super 35mm, I assume it could be more than double. Anyhow, my second question is, do you know of any optical reducer/converters like the speedbooster types from medium format to super 35mm, or at least 35mm? In case the crop is too much. Thank you!

Hey Sissy,

Ugh. I forgot to mention, in the above article, how bad at math I am! So, definitely check my work!

645 film dimensions: 56 x 41.5mm  (Ratio 8:5.2) (Diagonal 69.7mm)

135 film dimensions: 36 x 24mm (3:2) (Diagonal 43.27mm)

Super 35 film dimensions (varies): 25.34 x 14.25mm (on the Black Magic camera you mentioned) (16:9) (Diagonal 29.07mm)

645 to 135 format (full-frame) crop factor is: 0.62x

645 to Super 35 format crop factor is: 0.42x approximately...slightly different ratios...and I am not sure how that factors in. My calculations were based on the diagonal measurements.

I hope someone better at math than me can crunch the numbers in a more precise way!

As far as speed boosters for that conversion, there are definitely 645 to full-frame adapters, but I don't see any with optics...they are only straight adapters.

https://www.bhphotovideo.com/c/search?Ntt=Mamiya%20645%20to%20Canon%20EF&N=0&InitialSearch=yes&sts=ma&Top+Nav-Search=

We are looking for other options, but have come up dry so far.

I hope this helps!

hi. if i have an apsc specific lens for a cropped camera sensor say for argument sake its at 50mm focal distance. are these lenses designed so that 50mm focal distance gives a field of view that is a true 50mm or is it still 75mm? 

ie. 50mm apsc lens designed so that field of view is true 50mm not 75mm as you would get on a full frame lens. 

Hi john,

Good question. The answer is, "No."

A 50mm lens is a 50mm lens regardless of what kind of camera (or sensor) it is attached to. If it is sitting on a bookshelf, it is always a 50mm lens. The field of view is dependent on the sensor or film size. So, a lens that is made for APS-C cameras still has a focal length number is a fixed measurement.

So, a 50mm lens is a 50mm lens. 

Thanks for stopping by!

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