Canon EF 17-40mm f/4.0 L USM equivalent focal range on 1.6x sensor body

[Daniel Browning]

But, don't the effects of crop factor on apparent aperture depend on maintaining the same subject framing (as opposed to the same magnification)?

They do if you keep focal length the same. There are several methods to get the same subject framing. One is to keep the focal length fixed and vary subject distance with sensor size. That results in huge differences in perspective (and perspective distortion) and in many cases is not physically possible. Another method, which is the one I used for the comparison, is to keep subject distance fixed and vary focal length with sensor size. This results in the exact same subject framing no matter what the sensor size. As a further benefit, it does not cause any difference in perspective or perspective distortion.

You are cropping images taken with the same sensor, but in reality to get the same overall framing on the 1.6x crop that you have with your FF sensor, you'd need to move the camera further from the subject.

When focal length is fixed (e.g. 70mm on both), that is correct, but I was changing the focal length.

Using DOFMaster's calculator to 'recreate' your simulation, 5D2 70mm f/4 at 10 ft has a DoF = 1.47 ft. 50D 111mm f/6.4 at 16 ft has a DoF = 1.5 ft. (and if I could select 112mm as a focal length, which is 70x1.6, it would be 1.47 ft.).

Your recreation does not have the same subject framing, because you changed focal length and subject distance at the same time. If you wanted to see what happens when you change just subject distance and not focal length, here's what you would compare:

* 5D2 70mm f/6.4 at 10 ft
* 7D 70mm f/4 at 16 ft

That results in the same subject framing ("field of view"), but not the same angle of view and not the same perspective. The depth of field may not be exactly the same, either, because it depends on how close it is to the hyperfocal distance. (70mm f/4 at 16 ft is not very close, so the DOF is probably the same, but in other cases it may not be.) That's another reason why I prefer to scale focal length instead of subject distance.

In other words, as Bryan states, "...as a generalization, using a higher FOVCF DSLR will yield more DOF in your similarly cropped pictures because you will be farther from the subject."

That's correct. The same focal length and f-number combined with further distance results in deeper DOF for smaller sensors.

For example, if you have the 17-40 f/4 on the 7D and you shoot a portrait at 40mm and 5 feet, it will come out with a certain field of view and amount of distortion (big nose, small ears, etc.). If you take the same exact lens and put it on the 5D2 to shoot the same portrait, now you have to move much closer to fill the frame at 40mm. This difference in perspective results in extreme distortion -- much worse than 40mm on the 7D. That's why most photographers will use different focal lengths based on the size of their sensor. On their digicam, they'll shoot full length portraits with a 7mm lens. On 2X crop sensors, they'll use 25mm. On 1.6X, they use 31mm. On full frame, they'll use 50mm. If they used the exact same 50mm (or 7mm) on all sensors, they would get wildly different results.

In addition to perspective, there's also the fact that many times it's physically impossible to change subject distance. If you're shooting the moon with 800mm on 1.6X for example, then if you upgrade to 800mm on full frame the only option is to increase focal length, unless you have a rocket ship to take you closer to the moon. Similarly, if you're shooting a cramped room with 17mm on full frame and you switch to 17mm on a crop, you can't back up any further because you're already at the wall.

So while it's true that f-number does generally scale with sensor size when varying subject distance instead of focal length (with some DOF exceptions), I prefer to vary focal length because then all else remains equal.

Hope that helps. []