Maximum Magnification - different for EF lenses on crop bodies?

[neuroanatomist]

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?

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.

[Jon Ruyle]

So, you can't take the same picture

A technicality. Suppose then that you are using an MP-E 65 or else suppose that your subject is 35mm (then it is 1x on full frame and 0.625x on the 7D).

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.

John, if you'll excuse my saying so- and I mean this in the nicest way possible- you're being silly. You are comparing taking pictures with different sized subjects (by subject size I mean the linear size of the focal plane).


If I take one picture that is 15mm across (in the focal plane) and another that is 25mm across, of course I can get more DOF and less diffraction with the 25mm across picture. It doesn't matter which was taken with the small sensor and which with the large.


In other words, my point is just this: there is no *inherent* advantage of a large sensor over a smaller (or vice versa as some have claimed). In other words, assuming I have the right lens and I want to take a particular picture with a given DOF and as little diffraction as possible, I can ask myself, "what is the best sized sensor?" The answer is "it does not matter."

[neuroanatomist]

<span>John, if you'll excuse my saying so- and I mean this in the nicest way possible- you're being silly. You are comparing taking pictures with different sized subjects (by subject size I mean the linear size of the focal plane).

Jon, forgive me, but I have to say that I don't find it 'silly' at all. If you're taking a picture of an insect, you frame appropriately, and for one bug there's one 'size' to work with. Not all subjects are such that there is one appropriate size for the image. Here are a few examples:





In all four of those cases, a looser or a tighter framing would work. No, it wouldn't be "the same picture" but with a repetitive pattern that just means more or fewer repetitions are included in the framing, and with an amorphous subject, same idea. A reasonable approach with a standard macro lens is to simply shoot at 1:1 (or whatever mag is desired) - at least, reasonable to me, and I often shoot in just that manner - manually set focus at the MFD, then move camera+lens closer to the subject until a pleasing focal plane is acheived, and shoot. In that case, sensor size does matter, because at a fixed distance, the larger sensor delivers a deeper DoF and less diffraction - to me, those are often advantages.

[Jon Ruyle]

Jon, forgive me, but I have to say that I don't find it 'silly' at all.

You're right- you were not being silly. Sorry about that.

A reasonable approach with a standard macro lens is to simply shoot at 1:1 (or whatever mag is desired) - at least, reasonable to me, and I often shoot in just that manner - manually set focus at the MFD, then move camera+lens closer to the subject until a pleasing focal plane is acheived, and shoot.

I see your point. If this is your approach, the FF will give more DOF.


What I don't understand, though, is if you want a wider framed image with more DOF and less diffraction (like you would have with the FF camera), can you not achieve the same thing with the crop camera?


Those are four very nice pics, by the way. Is the first one a watermelon?

[neuroanatomist]

What I don't understand, though, is if you want a wider framed image with more DOF and less diffraction (like you would have with the FF camera), can you not achieve the same thing with the crop camera?

Sure, albeit with lower magnification (or by using a different lens, e.g the 60mm vs. the 100mm macro).

Those are four very nice pics, by the way. Is the first one a watermelon?

Thanks! Actually, the first one isn't even macro [:$], but it illustrates the point of freedom of framing (and in this case, scale, too!). Actually, it's the 'wake' left by a duck swimming across a duckweed-covered marsh, shot with the 24-105mm.

[mmodica]

I was talking about equivalent focal lengths when I said what I said - shoot a subject with the 60mm f/2.8 Macro on a 50D and the 100mm f/2.8 Macro on a 5D and look at the background and calculate the DOF - which is greater?