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, great article! 

This may be something easier to deduce than I'm aware, but somehow I can't figure this out: 

I'd like to know if the bokeh would be the same using a crop factor lens on a full frame camera, regardless of the same DOF effect. Basically, wondering if there is any benefit to investing in a crop lens with an aperture of 1.8 vs a full frame lens with an aperture of 2.8 if the bokeh of the 2.8 would be highly improved anyway by the use of a full frame body. I'm assuming a (larger) full frame sensor would let more light in regardless using either lens vs a crop sensor camera. 


Hi Elizabeth!

Thank you for the kind words!

Great question that I have a not super-clear answer for...

I will start with the brass tacks...I wouldn't ever recommend buying a crop sensor lens if you are already shooting a full-frame camera, regardless of that crop sensor lens's performance or specs. Yes, you can use crop sensor lenses on full-frame cameras, but, if you have a full-frame camera, I wouldn't invest in any crop sensor lenses.

The more complex answer is that depth of field is mathematical and you can calculate it knowing sensor size, focal length, focus distance, and aperture. Bokeh is the term for the visual rendering of out-of-focus highlights and, while it is affected by DOF, is more attributable to the lens's optical formula and aperture design. An f/1.8 lens might not have better bokeh (its all subjective anyway) than an f/2.8 lens.

Before I assign extra reading, be careful of the term "more light" when referring to crop sensor vs. full frame. Yes, a larger sensor will be exposed to more photons of light, but that light will not be brighter than the light falling on a crop sensor camera right next to it. In simple terms, this is why an f/1.8 lens is an f/1.8 lens regardless of what kind of sensor it sits in front of and why handheld light meters do not have separate settings for sensor sizes.

OK, your extra reading, if interested (no obligation!) is:

Understanding Bokeh: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/understanding-bokeh

Depth of Field Part III - The Myths - https://www.bhphotovideo.com/explora/photography/tips-and-solutions/depth-field-part-3

Thanks for reading and sorry for the long-winded reply (and homework)! :)



OK, now my brain exploded, and I know more about focal length, crop factor, and many other things than I ever knew I needed to know about.  I also have a question:  According to this illustration, an APS-C lens will produce a circular image when used on a full-frame camera.  What lens could I use on my Nikon D3200 (APS-C) to produce the same result?  I've been really interested in extreme fisheye photography but I long for the round images that I used to produce on film.  All of the super-wide lenses that I've found for an APS-C camera produce very curved perspectives with perhaps a bit of vignette in the corners, bot that's it.  

Hi John,

Sorry to explode your brain! Trust me, I wish this wasn't even a topic worthy of an article!

In general, all round lenses (APS-c or non-APS-C) produce round image circles. The camera simply captures a rectangular section of that round image circle and, on APS-C, it captures a smaller rectangle than a full-frame camera.

Nikon does make a DX fisheye lens—the Nikon DX 10.5mm [https://www.bhphotovideo.com/c/product/300487-GREY/Nikon_2148_10_5mm_f_2_8G_ED_DX.html]. I have that lens and it is pretty cool. I use it still on my FUJIFILM cameras (adapted) and it gives around 180-degrees field of view. You have to be careful not to get your feet in the shot!

I hypothesize that you could get the round effect you seek by using that 10.5mm fisheye on a full-frame camera making sure you don't use the automatic DX crop mode on the full-frame camera.

Unfortunately, I don't have a way to test this as I am not near the B&H Superstore, but, if you find it works, please let me know!

Thanks for reading!



PS. It looks like the 10.5mm is discontinued, but the "Imported" version is still in stock...so it might be time to move on that lens, if you can. Also, there are a handful of non-Nikon APS-C fisheye lenses on the market as well.

Hi Todd,

Really appreciated the article and come from that "old school" 35mm background myself, so I have struggled with exactly the kinds of translations you describe. However, I think I would take your description of the problem one step farther. I think it's not so much a bunch of dinosaurs trying to translate the modern world into their own language, as it is a bunch of dinosaurs and moderns trying to find a COMMON language. The problem, as I view it, is that the reference point has shifted. With the advent of digital photography and its many smaller format sensors, a "50mm" lens doesn't produce the same results -- the results we expect -- from camera to camera, or more correctly, from sensor to sensor (as you described so well). I think we're ultimately looking for a way to make the resultant image predictable, whatever name we slap on he lens. It doesn't matter if it's called a 50mm or a 105mm or an 85mm, we just want to know that when we see the designation, we have some idea what it's going to produce -- which unfortunately has become more complicated by the variety of sensor sizes. Just my $0.02. Best to all.

Hi Scott,

Thanks for the kind words!

I totally agree with your point and I think I alluded to it in the article above. The whole crop sensor equivalent-focal-length-field-of-view "thing" is an attempt to translate a physical measurement (focal length) into a field of view that is familiar to those who shot 35mm film cameras—probably, today, a pretty small part of the population.

In a simpler world, we might just describe lenses as "wide," "normal," and "telephoto."

Step back and think about someone new to photography who picks up a Micro Four Thirds camera. Do they really care what the full-frame equivalent of their chosen lens is? Probably not. They would probably be better off just being familiar with the field of view provided by whatever lens they are using...the focal length numbers would only be relevant when comparing two lenses in their camera bag..."This 12mm gives me a wider field of view than the 24mm."

More food for thought...by far the most used and popular cameras today are smartphone cameras. How painful would it be if we talked about the focal length, ad nauseam, of the iPhone or Android lens and tried to convert it into the world of 35mm film?

"It doesn't matter if it's called a 50mm or a 105mm or an 85mm, we just want to know that when we see the designation, we have some idea what it's going to produce -- which unfortunately has become more complicated by the variety of sensor sizes."...I couldn't have said it better!

Thanks for reading!



Newby here...everyone is always saying 85mm is best for portraits because it’s the most flattering.  So on APS-C they say say get a 50mm lens to account for the crop and then you have your roughly 85mm equivalent.  But isn’t this just field of view??   Shouldnt I still get a 85mm lens for my ASP-C  and just take a few steps back if I want to those flattering 85mm results??   I mean with a 50 won’t I just be getting more in the shot but the shot will still “look” like a 50” on a full frame.   I want the flattering portrait results.  I don’t care how much is in the frame 

In my opinion, yes, you are correct.  If you simply want the frame size on an APS-C sensor camera that is similar to an 85mm lens on a full-frame camera, then a 50mm lens (or 55mm) lens would give you similar framing.  However, you do not get the compression and face modeling from the 50mm lens as you would from an 85mm lens.  As such, I do exactly as you state and still tend to use an 85mm lens (or sometimes a 105mm lens) when using an APS-C sensor camera and simply take a few steps back to frame my subject as I would with the wider lens.  It may require me being 6-12 feet farther from my subject, but the benefit of the compression and slight flattening of the features from the longer focal length in the portrait range is worth it in my opinion.  I always recommend to test both options for yourself and see which you prefer for your shooting style, but while I have both the 50mm and 85mm lens and I use my 50mm for candid images, I still prefer my 85mm lens for portraiture usage needs.

Hi, thank you for this article, can I throw a spanner in the works

I have Olympus OM-D1v2 .

I have a 30mm Olympus macro lens 

I have also a 28mm Tokina legacy lens.

Here is my conundrum.

I take the same pictures with both lenses, both are near identical.

So is a dedicated Olympus m43 lens designed for the the camera a 30mm lens and why does the Tokina 28mm lens give a slightly wider picture?

Look forward to your reply

Hey Chris,

I welcome spanners and all other types of wrenches into the works!

Good question and I will preface my answer with a disclaimer that my "answer" is just a hypothesis...

Strangely, in the lens world, not every focal length is measured the same (or accurately). My guess is that if you could figure out the nodal point of the lens and used a ruler to measure from there to the sensor or film plane, you would find that the ruler and the focal length engraved on the lens are different.

This is proven by the fact that if you look at, for example, different Nikon F-mount 50mm lenses, they have slightly different angles of view (1-degree differences).

You might want to look up the specs for that Tokina lens and compare it to the published 40-degree angle of view of the Olympus lens to see what you find.

Let me know what you discover and if we have to throw more wrenches into the gears!



Thanks very much. Your explanation is very clear and concise. I always understood it to be, i.e. a 70-200 mm FX lens to be a 105-300mm on a 1.5 AP-C or DX format. The only question I have is why does this not seem to follow suit when I have a 12-24mm DX lens and if I were to put this lens on a FX format why it would then be 18mm-36mm. It seems this concept gets reversed some how, or I still have confusion.

I had always figured if I used a 300mm fx lens on my DX 1.5 crop AP-C that that same lens would have a 450mm effect or magnification on my DX camera? Is this correct or am I still confused, or does this concept change with zoom lenses? Could you please clarify this if I am mistaken? Thanks! Michael Breskin

Hi Michael,

Thank you for the kind words!

Great questions and, trust me, you aren't alone in this confusion.

First, to help clear things up, there is no such thing as an "FX" lens. There are 35mm format camera lenses and then there are APS-C (DX) lenses and Micro Four Thirds and others. This is a common confusion that leads to other confusion.

So, a 70-200mm lens on an FX camera gives a field of view commensurate with a 70-200mm lens. Put that 70-200mm lens on an APS-C DX camera and the camera only captures a part of the image circle that matches up with the field of view of a 105-300mm lens.

Your first question:

If you put a 12-24mm DX lens on an FX camera, it is a 12-24mm lens on that FX camera. The field of view of that lens will be that of a 12-24mm lens. The rub is that the DX lens projects a smaller image circle than a non-DX lens and that image circle is not designed to cover a 35mm-format "full-frame" FX sensor. This is why you get vignetting when using DX lenses on an FX camera.

The concept is not reversed. Crop factor does not apply when putting a DX lens on an FX camera.

Your second question:

Yes, a 300mm lens (notice that I did not say "300mm FX lens"...its just a 300mm lens) on an APS-C DX camera has a field of view that is the same as a 450mm lens on an FX camera.

Your third question:

Zoom lenses do not change the math...focal length is the factor, not if the focal length is variable.

I hope this helped clear up some of the confusion and answered your questions. If not, hit me up with another reply!

Thanks again for the kind words on the article and thanks for reading!



Todd thanks very much for the clarification.


You are very welcome, Michael! Thanks for stopping by!

Fascinating article, Todd. Reading it inspired me to conduct an experiment. I photographed the same object twice with my Canon 800D (Rebel T7i), first with a crop sensor lens and then with a full frame lens. For the first shot, I used a Sigma 17-50mm F2.8 EX DC OS HSM set to 50mm. For the second one, I used a Canon EF 50mm f/1.8 STM. I was expecting both images to have the same field of view. However, I found that the image taken with the Canon lens is 13.48% more zoomed in. Is it possible that the Sigma is just a bit wider at its wider setting than they advertise? Or does the fact that one lens is for crop sensor and the other is for full frame sensor affect the equivalent focal length?

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Hi Richard,

Thanks for the kind words!

Interesting results from your experiment!

I am surprised the difference was as large as you report. Certainly, there will be a difference in field of view between a Sigma zoom at 50mm and a Canon prime at 50mm. You might even find a slight difference between two Canon primes as you'd be looking at lenses of different optical designs.

So, yes, you discovered that there might not be an exact standard for 50mm between lenses/manufacturers which is ironic as 50mm refers to a measurement! But, my guess is that you would see the same results between the Sigma and Canon on a full-frame camera (minus the vignetting of the APS-C-specific lens) and that the difference is not due to the crop-sensor lens design but more aligned with your first question that it is a bit wider than advertised at "50mm."

I am now trying to figure out what two lenses I can compare on my Fujifilm X-T3 to see if I get similar results!

Do you have other lenses you could test just to deepen the mystery? :)



I have a few questions that I think pertain to crop factor, but if not I sincerely apologize in advance.

I want to build a camera for extra-wide panoramic views. My idea is to build a simple brownie-type box camera that uses 120 film with a negative size of 2.25" by approximately by 15-16" and use a large format lens. If I was to use say a 215mm for an 8x10 or 11x14 camera, how would that equate to an equivalent focal length in medium format? Is there a formula to calculate the image circle of a lens so I don't have a vignetting effect at the ends of the negative?



Hi Matt,

Whoa...while I think you are in the right "crop factor" place, I will hold a grudge for having you make me do math. [And, if you saw my college transcripts, you might decide that I am not the right person to answer!]

Actually, I am going to punt. I don't want to cut-and-paste someone else's hard work, but if you do an internet search for "How to calculate the image circle of a lens?" you will get a handful of good results.

Or, you can buy an Hasselblad X-Pan :)

To check your math, the thing to remember is: the larger the camera's sensor or film, the greater the focal length needs to be for a given field of view.

135 Format Film - 50mm gives the "normal" field of view.

APS-C - 35mm = 50mm field of view

Full-Frame - 50mm = 50mm field of view

Medium Format - c65mm = 50mm field of view

Large Format - c135mm = 50mm field of view

I hope some of that helps! If not, feel free to come back!

Thanks for reading Explora!



I am just starting to peek through the wormhole of astrophotography and have just started trying to understand Crop factors compared to Field Of View regarding sky images. I am experimenting with both a Nikon 1 J3 (2.7 CF) and an iPhone 7 (7.21 CF, f/1.8, FL 4(28)mm-8(56)mm). Both will be used for Prime Focus and Eyepiece Projection photography. With the Nikon 1, I will try some imaging with just the lenses and tripod also. The Nikon1 I have the following lens: 1 Nikkor VR 10-30mm f/3.5-5.6; 1 Nikkor 11-27.5mm f/3.5-5.6; and from my film days, a Hanimex 80-200mm f/1.4-XXX. I am also considering purchasing the 1 Nikon 10mm f/2.8 for aside angle "Milky Way" lens. If I understand your explanations correctly, the Crop Factor-FL calculation is just giving me and idea on the apparent FOV that changes with the different sensor sizes. If so, then am I correct in that this will not change my exposure calculations in regards to ISO, Shutter Speed, and exposure time? How does this FOV change relate to the telescope focal length-eyepiece magnification calculations then? In either prime focus or eyepiece projection photography the Nikon1 camera lens does not play a role so the CF is just relative to the telescope FL & f ratio, just a narrower field of view, right? The iPhone however will shoot through an eyepiece, which, depending on the one used, has a FOV calculation of its own and then narrower CF 7.21X of the iPhone. Will my exposure calculations need to be altered? Im trying to decide if I need to create a cheat sheet chart that recalculates the aperture settings, ISO adjustments and exposure times for the virtual focal length changes of the various sensors or is the light gathering power the same across the board and the only change will be in the overall FOV of my image? Thanks, Mark 

Hi Mark,

Thanks for your questions. I hope I can give you good answers here!

As you probably know, the Nikon 1 sensor has a 2.7x crop factor when compared to the 35mm-format full-frame sensor. Your 10-30mm lens on the Nikon 1 gives a field of view equivalent to a 27-80mm lens on a full-frame camera.

You are correct...exposure calculations, ISO, shutter speed, etc, are, for practical purposes, unchanged by sensor size. This is why handheld light meters do not have a "sensor size" setting.

You are the first to ask questions regarding prime focus and projection photography, so I have had to put on my thinking cap (helmet, actually) and here is my attempt:

Prime focus—I believe crop factor is still at play here. When I digiscope with my Leica spotting scope through a camera adapter (not through the eyepiece), I get more "magnification" with my APS-C cameras than I would with a full-frame camera. I put "magnification" in quotes, because that is actually a misnomer. The smaller sensor captures a smaller portion of the image circle and that is why it looks like we have magnified the image.

Eyepiece Projection—I have not really done much of this aside from rudimentary iPhone digiscoping, but think that crop factor would come into play here only if your sensor is not capturing the entire image circle projected by the eyepiece. If the eyepiece image circle is falling completely inside the view of the sensor (you see heavy vignetting around the image), then there is no crop factor at play.

Exposure adjustments—I mentioned above that sensor size does not change exposure calculations. This is correct. However, if you visualize an eyepiece projection where the iPhone or Nikon 1 sensor is seeing more or less vignetting around the image circle, that will definitely change the exposure as the camera, if not put onto spot metering, will try to compensate exposure for the dark areas of the image.

In the digiscoping you are describing, aperture is not in play. Your only real exposure consideration is the shutter speed needed to keep the stars from trailing for your chosen focal length (even if using a tracking mount). With that hard limit, you then adjust ISO to compensate for under/overexposure, as needed. Crop factor does not come in to play here. Your metering/exposure is what it is.

I hope that was the explanation you were looking for, Mark. Standing by for follow-ups!



Thanks Todd. That helps a great deal. Where I was getting confused with the crop factor was in trying to calculate maximum length of exposures based on the Rule of 500. The way I understand it, I divid 500 by the focal length of the lens to get the maximum exposure time before stars trail. However, this is for full frame sensors. I was then dividing the time of exposure for the FF sensor by the crop factor of my Nikon 1 of 2.7 and my iPhone 7, for digiscoping, 7.21. In essence, I was reducing my exposure time for the smaller sensor. But if I understand your explanation, the sensor sees what it sees. It’s vision may be narrowed by the crop factor, but the focal length and aperture are unchanged (amount of light reaching the sensor is the same regardless of the size of the sensor). So, when calculating my maximum exposure time limits with the lenses I have with my Nikon 1 (10-30mm VR f/3.5-f/5.6 & 30-110mm VR f/3.8-f/5.6 and an adapted Hanimex 80-200mm manual f/4-f/22), I only need to divide 500 by each focal length? Or do I divid the quotient by the crop factor? Or should I just start with trial and error? 

Thank you for your help on this. Best Regards, Mark

Hey Mark,

Great follow-ups there.

First of all, the Rule of 500 is not quite a rule, but a guideline (to quote a semi-famous movie). :)

Yes, you can use it as a starting point, but crop factor, due to the reduced field of view, is definitely in play here.

In this article [https://www.bhphotovideo.com/explora/photography/tips-and-solutions/backyard-astrophotography-part-3-capture] I said the following:

"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."

Use the 500 or 600 "rule" as a starting point and adjust from there. There are other factors at play, including the declination of your target stars, but the math gets super fun at that point!

You are very welcome for the help! Are you posting the photos anywhere we can see them?



I have a question ,

for wild life photography many say that crop sensor cameras are better for wild life , they suggest that if for examples  use 400mm lens on the crop sensor it would bring us 640 mm focal length , and from what the aforementioned description tell us here ,  this is only the feild of view of that lens on the cropped sensor camera .

My question is that if I shoot with 400mm on the fullfarme camera then crop to the picture the same as the cropped sensor picture , the quality of that final picture is the same as the cropped sensor or not ?

That will depend on the exact camera and how many MP it has.  If it is a high mega pixel camera, like the A7RIV which is 61MP, shooting at 400mm and then zooming in may look even better than an APS-C sized camera with a lens cropped.  However there is no specific testing on this.

Thank you for this article. Just so I am understanding this correctly...what I think you're saying is that a 50mm lens on an APS-C sensor Canon camera (1.6 crop factor) would have an equivalent focal length of 80mm. However, I've heard that 50mm is the closest to what we see with our eyes, and is good for filming talking head videos, etc. So if that is correct, I need to get something close to a 30 mm lens on an APS-C sensor camera so that it looks like a 50mm view (to get my talking head to look the closest to what I would expect looking at them without a camera)? Is that correct? Please advise. Thank you.

That is correct.  To get the 50mm field of view you would want to look at lenses around the 30 to 35mm Focal Range with an APS-C camera.

Will my MFT lens cover more area on the z cam super 35 sensor than the area on a MFT sensor? the Z cam  E2 S6 comes with a MFT  adapter and I was wondering if a MFT lens on the z-cam covers more sensor area on the Super 35 sensor?

MFT can be used to describe both a mount kind and a sensor size.  However they are not always one in the same.  For example, there can be MFT mount lenses that cover a Super 35mm Sensor.  Most MFT lenses though cover a Micro Four Thirds Size Sensor which is smaller than Super 35mm.  It  will all depend on the exact lenses you have in this case.

This has been bothering me forever, and my non-photography inclined roommate just asked. I couldn't provide him with an answer, and google has only been so helpful. So I'm turning to the pros :) All things being otherwise equal, FF vs APS-c with the same megapixels, same exact positioning, etc. Using the same lens from the FF on the APS-c will have an effective 1.5x zoom factor. What I get stuck on is how this does not also reduce the quality of the image. Like digitally cropping the image (in my head, anyway) SHOULD effectively be exactly the same as using a crop sensor. Cropping away the extra light that it's not able to use. But instead, you get the same quality image, just zoomed in an extra 50%? I'm sure there's a good math explanation. I just don't have it handy.



Jarad, you are certainly asking the right questions here. This is unfortunately where it gets a little more complicated.

So, you are correct. All things being equal the only difference is the 1.5x crop. However, in the real world, you are likely comparing a 24MP full-frame sensor to a 24MP APS-C sensor (two fairly standard options today). In this case, the difference becomes low-light performance and dynamic range. Since the 24MP APS-C sensor is cramming more photosites (pixels) onto a smaller surface area, each pixel is smaller and therefore more likely to generate noise. Simplified, a crop sensor is more likely to show noise and reduced dynamic range at the same ISOs compared to a full-frame sensor of the same resolution. 

The WOULD be a difference in pixel count (therefore, image quality) in this example. If you shot using the same lens, (let's say a 50mm) on both a FF camera and an APS-C camera with the same 24mp sensor, then the result would be: FF camera has it's "50mm-looking" image, at 24mp. APS-C camera would yield a "75mm-looking" image at 24mp. So now, if you want to crop in on the FF image, to make it look like the same 75mm-looking image, you can do it, but it will result in throwing away like, 1/3 of the resolution to do so, so you end up with something more like a 16mp image.

Hi B&H team.  Thanks for the helpful article.  I am a little confused on just one thing.  I recently purchased the a7r4 with 61 MP.  If I want to get closer with a prime, is it better one way or another to use the aps-c mode on the camera or crop after the fact?  My question is why ever use the crop factor if you can just crop in post? (aside from file size)  Sorry for the question if the answer is obvious.  :-)

If you center-crop the image to a 1.5x crop during post-processing while editing your image, it would give you an identical effect to using the APS-C Crop Mode internally in the camera.  The effect would be the same.  The main benefit of the internal APS-C Crop Mode is when using a lens designed for an APS-C sensor camera so there is no vignetting when using the lens.  However, the benefit of using the APS-C Crop Mode feature with a full-frame lens is if you are sure you want the image cropped beforehand, and/or if you need your current lens to have a tighter framing from your current working distance; it would save you time from positioning and cropping the image using your image editing software.  If you are using it on one or two images, you shave a few seconds off of your editing time.  If you are shooting tens or hundreds of images, you can shave off minutes (or more) of your computer editing time.  The benefit of not using the APS-C Crop Mode when using a full-frame lens is you get to choose the amount and the precise position of crop you want during post-production.

I have a few lenses from my medium format camera that I wish to adapt to my MFT sensor camera. Will this simply be  2x the focal length of the medium format lens or does the format of the lens come into play? 

Hi Yash,

Yes, you would simply multiple the current focal length of your medium format lens by 2X when using it with a Micro 4/3 camera. 

Hey I was looking at the Pictor 20-55 lens for my pocket 4K with metabones .71. It’s an APSC lens so I’m wondering do I still do a 2 times crop for the 4/3 sensor to FF eg 20x2x.71 = 28mm or since it’s APSC the crop is smaller? And do something like converting 4/3 to APSC and then for E.g.  20x 1.4x.71 = 20 mm?

If you are trying to find the full-frame equivalent focal length for a lens used on the Blackmagic Design Pocket Cinema Camera 4K, then as the lens uses a Micro Four Thirds sensor, ALL lenses used on this camera would require you to multiply the focal length by 2x to find its full-frame equivalent focal length.  If you are using the Metabones T Speed Booster Ultra 0.71x Adapter for Canon Full-Frame EF-Mount Lens to Micro Four Thirds-Mount Camera, you would also need to multiply the result by 0.71x to find the equivalent full-frame focal length when used on your Blackmagic Design Pocket Cinema Camera 4K.  As such, the DZOFilm Pictor 20-55mm T2.8 Super35 Parfocal Zoom Lens used with the Metabones T Speed Booster Ultra 0.71x Adapter would be the equivalent of a 28.4-78.1mm lens used on a full-frame sensor camera.  (20mm x 2 x 0.71 = 28.4mm, and 55mm x 2 x 0.71 = 78.1mm).

 I recently purchased a MFT Cinema Camera and wanted to see if someone can clarify some focal lengths vs. crop questions. I understand the crop factor for MFT is 2X / APSC is 1.5 and so on but my question is visually does the crop manipulate the image to the cropped size or does the image remain equivalent to the original focal length? In other words, does a 16mm MFT, cropped to a 32mm equivalent, remain looking like a 16mm full frame in the image or does the image look like a 32mm full frame ? Im asking this because when I film a close up, I want the image to look as natural as possible as if filming with a 50mm full frame prime. In this case would I get a 25mm MFT or a 50mm MFT even though it is cropped to 100mm. Can someone enlighten me?

The “equivalent” focal length is your guide for what the image will look like. So, a 25mm lens on MFT will be cropped to a 50mm equivalent, which will give the same field of view of a 50mm lens on full-frame. Does that help?

So if I have a film-era 70-210-mm lens, when I put that onto my Pentax K10D, it will behave as a 105-315-mm lens in terms of field size but not 1.5x more magnified, right?  The magnification will remain the same?   (So, for example, the moon will not look 1.5x larger with the DSLR -- just less surrounding sky, right?

Yes your 70-210mm lens would act more like a 105-315mm lens due to the 1.5X crop, so the magnification would be increased. 

Thank you Todd for this brilliant article.  Zuiko Lenses for Olympus and the Micro 4/3 system have long been the subject of attack mainly by their competitors to undermine their great and at times unequalled performance particularly in Aperture size.  So basically you would find people that would say that a lens with 300mm F2.0 while it would be equivalent to 600mm, but is not really f2.0 but rather f2.0 x a crop factor of 2x = F4.0, and that is just flawed as you’ve stated in your article.  But I feel it needs to be cleared up and spelled out even more.

This has gotten so bad, that it has effected Olympus sales, leaving the companies future in jeopardy.  That would be extremely sad, as those little gems are a product of sheer genius when Olympus / Panasonic/ and Leica decided with the advent of Digital sensors not to subject themselves to the limitations of 35mm cameras and be liberated once and for all to create more practical products and with even greater abilities. 

Thanks, Zayed!

Ahhhh...a fellow cropped-sensor fan! Are we the only ones left? :)

While you could make an argument that more photons can strike a bigger sensor, if it really affected the actual aperture, then there would be a sensor size setting on hand-held light meters!

I have always enjoyed my time with Micro Four Thirds cameras and lenses and the Olympus glass and Leica/Panasonic glass is spectacular. It is a shame that the marketing folks have blatantly tried to make those who shoot smaller-than-full-frame sensors feel inadequate. If sensor size was truly the key to great images, we would all be clamoring for large-format digital and scoffing at those who were shooting medium-format digital...much less those puny little full-frame sensors! And, forget the smartphone camera, too! Impossible to get a good photo on that tiny sensor!

Join me while we keep fighting the good fight and making great photos with inferior cameras! :)



If you mount a full frame EF mount lens on APS-C camera with an EF to EF-M adapter is it still subject to the crop/multiplication factor? I'm trying to figure out what prime EF lens would be 35mm equivalent on my Canon M50.  

It would still be subject to the crop factor, in this case 1.6x. Crop is dependent on sensor size, though some very specialized adapters referred to as Speedboosters or focal reducers can actually counteract this. 

To get a 35mm equivalent on the M50 I would actually recommend the native EF-M 22mm f/2 Lens. No adapter required. If you do want a full-frame EF lens for other reasons I would recommend the EF 24mm f/2.8 IS USM. If you don't need a full-frame lens, the EF-S 24mm f/2.8 STM is another option dedicated to APS-C.

That answers my question perfectly, thanks. I understand that the diagonal of a different sized sensor is proportional to a standard sized sensor, but wouldn’t the width of the adapter increase the focal length causing more surrounding area of the image to hit the sensor (like a wider angle shot)? I'm curious how this works, but I really started this because I want to try a Cine lens on the M50.

Great question! So if you compare a DSLR (with the Canon EF mount) and a mirrorless (with the Canon EF-M mount) you will notice that the DSLR is a lot thicker. The reason is that the optical system in the DSLR has to account for a mirror, so the distance from the mount to the sensor is longer.

What an adapter does is basically make sure that the DSLR lens is the same distance from the mount to the sensor as it would be if it was mounted natively on a DSLR. Make sense? So the lens is designed to work at the distance the adapter puts it at, meaning no change compared to if it was on a DSLR.

Here's an article that goes into way more detail on mounts and adapters: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/introduction-to-lens-mounts-and-lens-adapters

Please let us know if you have any more questions!

I get it now! In my train of thought, I was mistakenly putting an APS-C lens to an APS-C sensor with an extension tube in between.

Is there any advantage in terms of image quality for the smaller sensor to crop the image, compared to a full frame, versus cropping a full frame image down using photo editing software? 

Maybe. It would be on a case-by-case basis. If you find yourself always using a crop mode or cropping in you may want a 24MP APS-C camera instead of a 24MP full-frame sensor since you'll have more resolution to work with in the end. However, that extra resolution may come with more noise in low-light conditions. So there is always a trade-off.

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