Quote Originally Posted by Jon Ruyle
To compare apples to apples (ie, compare taking the same picture with two different sized sensors), you should compare 1.6x magnification on the 5DII with 1x on the 7D.

How do you do that with a typical 1:1 macro lens?


Quote Originally Posted by Jon Ruyle
So the question should be, suppose I want to take the same picture of a given small subject (eg, the picture is 15mm across along the focal plane) and I want to stop down to get a given DOF (say 2mm). How much resolution do the rules of diffraction allow? The answer to this question is independent of sensor size. Ie, if you stop down more with the FF camera so that DOF is the same, diffraction limited resolution will be the same (by this I mean the airy disk will be proportional to sensor size, giving resolution which is independent of sensor size).

Ah, but with your 15mm subject, that's smaller than an APS-C sensor. So, assuming one does not have an MP-E 65mm (which I know we both do), 1:1 magnification of your subject would approximately fill the frame vertical dimension of APS-C at 1:1. But, since you'd already be at the MFD for 1:1, you can't fill the FF sensor. In practice, then, with a 15mm subject one would most likely just get that subject as large in the frame as possible, and it would just fill less of the frame on FF. So, you can't take the same picture - the FF sensor will always show more background than APS-C in that scenario, right? And if that's the case, diffraction-limited resolution would not be the same - subject distance would be the same, and the FF sensor would out-resolve the APS-C, and have deeper DoF, too.


Now, if you backed the APS-C camera off so the 15mm subject filled the same proportion of the frame as it would on FF, to 'take the same picture', then you're correct. But I would argue that one wouldn't do that except in a contrived situation, unless you have a subject that is larger, say, around 24mm, so you'd be shooting at 1:1 on FF, but 0.625:1 on APS-C to take the same picture. In that case, the diffraction-limited resolution would be the same for both sensor formats.