Is it beneficial to use a crop sensor camera with a full frame telezoom?

Question

I have a Canon 100-400L telezoom lens. It gives up to 400mm of focal length, but when used on a crop sensor camera, the equivalent focal length would be 640mm.

Because the equivalent focal length would be 1.6x larger on a crop sensor camera, I should according to the theory get 1.6² = 2.56 times more pixels if using the lens on a crop sensor camera with equivalent megapixel count, when compared to a full frame camera.

So, is there any truth to this? Is there a large benefit in using a telezoom lens designed for full frame on a crop sensor camera to get more reach?

Answer 1

By using an APS-C camera with higher pixel density, you will get "more pixels on the subject" than with a FF camera with lower pixel density.

Sometimes that can be a good thing.

There are many wildlife photographers who use APS-C cameras with telephoto lenses for this reason. It allows one to get more "reach" with shorter (and less expensive) lenses without taking the light loss penalty of a 1.4X or 2X extender.

Take, for example, the Canon EOS 7D Mark II. It has 20.2 MP in a 22.4 x 15.0 mm APS-C sensor. A 4.09 µm pixel pitch equates to 51.7 MP on a 36 x 24 mm full frame sensor. Interestingly enough, that's just slightly more than the Canon EOS 5Ds/5Ds R with 50.6 MP.

So if one were comparing the 20.2 MP 7D Mark II to the 50.6 MP 5Ds, there would be very little difference in terms of how many pixels the same subject at the same distance seen through the same lens would occupy on either sensor.

What would be different:

• Cost: The 20.2 MP 7D Mark II sells for well less than half of the cost of a 5Ds
• Speed: The 7D Mark II can shoot 10 frames per second for up to 31 raw images or as many jpegs as the memory card can hold. The 5Ds/5Ds R can only shoot at 5 fps for 14 raw or 510 jpegs before bogging down. This is mainly because of the increased processing load of 50.6 MP images versus 20.2 MP images.

On the other hand, if one is comparing the 20.2 MP 7D Mark II to something like the 22.3 MP 5D Mark III, then there would be a noticeable difference when viewing images at 100%. There would be a 53% (1.53X) advantage linearly, and a 2.33X advantage in terms of area.

In the end it comes down to the pixel pitch of each camera, regardless of the total size of the sensor.

As for lens performance of the same lens on an APS-C camera versus on a FF camera cropped to the same dimensions of an APS-C camera: There would be no difference whatsoever. Both images would be using the same exact part of the image circle cast by the same lens. Assuming the resolution of the lens is not the limiting factor, then the sensor with more pixel density (smaller pixel pitch) should give better performance.

The original EF 100-400L IS is a pretty good lens but might be the limiting factor with very dense sensors such as the 7D Mark II or the 5Ds. The EF 100-400L II IS is enough sharper that it should not be the limiting factor with any of Canon's current sensors in EOS DSLRs or EOS R mirrorless cameras.

Answer 2

The answer is: perhaps, but not as much as you'd think. Full frame lenses are at their best in full frame cameras. While they can be used for crop sensor cameras too, they are not optimized for this use. So, the benefit may be there but it's not 2.56 times more pixels.

Let's take a look at the lens on DxOmark.

DxOmark for 100-400L IS USM lens gives these perceptual megapixels (P-Mpix) counts:

• 760D: 7 P-Mpix (24 Mpix sensor)
• 5DIII: 13 P-Mpix (22 Mpix sensor)
• 5DIV: 14 P-Mpix (30 Mpix sensor)

Considering the smaller sensor size of 760D, one needs to do a 1.6x crop (in length units), or 2.56x crop (in areal units) on the full frame 5D figures to obtain the same area. Thus, the equivalent figures for 5DIII and 5DIV are 5.1 P-Mpix and 5.5 P-Mpix. Normalizing for sensor megapixel count (24 Mpix is 75% of 22 Mpix + 25% of 30 Mpix), we get 5.2 P-Mpix on full frame with a 24 Mpix sensor.

So, yes, a crop sensor camera has some advantage here but not as much as you'd expect. 7 P-Mpix / 5.2 P-Mpix = 1.35 (approximately), so instead of getting 2.56 times more perceptual pixels, you get only 1.35 times more perceptual pixels.

Furthermore, a practical detail: when trying to photograph flying birds, it's hard to get the bird to the viewfinder with a 250mm lens on a crop sensor camera because of the long focal length. A 400mm focal length would be even more insane, making it hard to aim the camera. So, a full frame camera would probably be a better choice to use with this lens, when photographing flying birds: aiming the camera is somewhat easier, and the final photo can be cropped at a minor cost in perceptual pixel count.

However, when taking photos of the moon, a tripod can be used. With a 250mm lens on a crop sensor camera, one can clearly see the rotation of the Earth, but this is mainly a minor annoyance rather than a major problem when taking multiple photos of the moon. A 400mm lens on a crop sensor camera would not be a problem in this case, and the 1.35 times more perceptual pixels would create a slightly sharper image.

If the full frame camera is a mirrorless one (such as EOS RP), there's always the option of using a teleconverter. Autofocus doesn't work with DSLR + teleconverter unless the aperture number is at most 5.6 (there may be some exception cameras to this rule allowing f/8 autofocus), but on a mirrorless camera, f/11 should work with autofocus.

Moreover, one more thing to consider: the 100-400L lens, like most lenses, is sharpest in the center of the picture. I didn't take this into account when calculating the 1.35 times benefit. It may be the case the 1.35 times benefit diminishes to equality when cropping at the center of the frame, which is the sharpest location.

An interesting observation: the cheap 55-250mm has on the 760D 8 P-Mpix, but the focal length does not go up to 400mm.