Crop factor

Does size matter?

One of my goals for my recent camera upgrade was to jump from a ‘crop-sensor camera’ (like the Nikon D500) to a ‘full-frame camera’ (like the Canon R5). I had long avoided the full-frame cameras like the Nikon D800 and D850 because they (and their lenses) were so much larger, heavier, and costlier… but I favor smaller and lighter equipment because I like to be able to hike with them.

The switch from DLSR to mirrorless, however, enabled me to get a full-frame camera body (Canon R5) that was actually lighter (738g) than the crop-sensor camera body (Nikon D500) that I had been using (860g). To be fair, though, the R5 with its 24-105mm kit lens was heavier (1438g) than the D500 with its 16-80mm kit lens (1340g). But, I rationalized, it’s full frame! better quality, right?

But what else was I gaining… or losing? People often say that crop-sensor cameras have better ‘reach’ – because their effective focal length is a multiple of the lens focal length, making subjects appear ‘closer’. The truth is much more complicated. Read on.

Canon 500mm; 1/640 at f/7.1, ISO 2000

First, let’s compare the camera bodies.

Canon R5Nikon D500
Infomanual pagespecs
Crop factor11.01.5
Image size28192 x 54645568 x 3712
500mm lens reach3500750
f/4 lens is effectively…44.06.0
100 ISO is effectively…5100225

The notes below correspond to superscript notes above. Crop factor is surprisingly complicated, but the page (1) is extremely helpful and the videos (4,5) even more so.

  1. What is Crop Factor? (
  2. One forum said 5088 x 3392, but I calculate it as 5120 x 3415
  3. in terms of “field of view”
  4. Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses (Tony Northrup)
  5. Crop Factor with ISO and aperture (Tony Northrup)
    1. Crop Factor Part 3: Responding to Critics, Corrections
    2. Crop Factor 4: Debate with a Critic & New Sensor Performance Data

The Canon R5 is a full-frame camera (crop factor 1.0), whereas the Nikon D500 is a crop-frame camera with crop factor 1.5. That means the D500 sensor is smaller than the traditional 35mm full-frame sensor, so it is capturing a smaller portion of the scene (the diagonal is 1.5x smaller). When I use the new Canon R5 body, at the same focal length, I will capture a wider field of view… but if I’m photographing a distant bird, that doesn’t help. I’ll just crop the photograph anyway! But let’s take a closer look. Suppose I put the Canon R5 into ‘crop mode’, which just means the R5 will crop the photo for me, in-camera. It uses a crop factor of 1.6:

Canon R5Canon R5 cropped
Crop factor1.01.6
Image size8192 x 54645120 x 3415
500mm lens ‘reach’500800
f/4 lens is effectively…4.06.4
100 ISO is effectively…100256

Ahah! so the Canon R5 in crop mode is not all that different than the Nikon D500, in terms of resolution (17.6mp vs 20.9mp), reach (800mm vs 750mm), f-stop (f/6.4 vs f.6.0), or ISO (256 vs 225). But let’s be fair, and use the same 1.5 crop factor, e.g., if I were to crop the R5 photo in post-processing:

R5 at 1.5cropNikon D500
Crop factor1.51.5
Image size5462 x 36435568 x 3712
500mm lens ‘reach’750750
f/4 lens is effectively…6.06.0
100 ISO225225

They are now nearly identical. So the conclusion is that both cameras give me effectively the same resolution, f-stop, and ISO, for a given ‘reach’, when photographing something relatively small. But when I want that larger field of view, I can take full advantage of those beautiful 45 megapixels, lower f-stop, and lower ISO… not possible in the crop-frame camera.

Now let’s look at the lenses I used to have in the Nikon (left column), and see how they compare to the lenses I have on the Canon (right column). The center column calculates the ‘effective’ focal length of the Nikon lens, given the 1.5 crop factor.

NikonNikon effectiveCanon
70-200 (Sigma)105-300100-500

They’re remarkably similar, in many ways. And if I add the 1.4x teleconverter:

CanonCanon with 1.4TC

I can get even more ‘reach’, at the cost of an additional stop of light.

Compare the following two photos, both taken with a 500mm lens; the first with the full-frame Canon R5, the second with the crop-frame Nikon D500.

Canon 500mm; 1/640 at f/7.1, ISO 2000
Nikon 500mm; 1/200 at f/5.6, ISO 500

Notice the narrower field of view in the latter, crop-frame image.

Summary. My new full-frame kit gives me much better resolution (45mp vs 21mp) when I can fill the frame with a subject, and about the same resolution when the subject won’t fill the frame and I want to crop (20mp vs 21mp) as often happens for distant wildlife. And, depending on the lens, about the same effective aperture or ISO. And, depending on the lens, it’s not much heavier. And, I’ve gained many other features of a modern camera – double or even triple the frame rate, faster memory cards, and vastly improved auto-focus system. I’m looking forward to exploring its capabilities!

Author: dfkotz

David Kotz is an outdoor enthusiast, traveller, husband, and father of three. He is also a Professor of Computer Science at Dartmouth College.

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