Maximum Magnification - different for EF lenses on crop bodies?

I was just wondering, the MM spec for EF lenses, would that be different if the lens was mounted on a 1.6x crop body for example? I've been studying the diagrams and formulas in this old photography textbook I picked up, but I can't seem to figure out for sure how crop factor affects MM (if at all). Anyone know?





Thanks!


~Matt M

1.6 x focal length equals the focal length you would need on a FF body to get the same Image scale and field of view (magnification)as a 1.6 crop body.
1.6 x 100mm = 160mm, a 100mm lens on a crop body is equal to a 160mm lens on a FF Body.


Or if you have a FF body, focal length divided by 1.6 is equal to the focal length on a 1.6. Crop Body.
200mm / 1.6 =125mm, a 200mm lens on a FF is equal to a 125mm on a 1.6 crop body


IOW, with a FF body you would need a longer lens to get the same image scale and field of view that you could get with a crop body using a shorter lens.

MM is the same with any sensor.


Magnification is the sensor size divided by image size. Cropping changes both by the same factor, thus the magnification is not affected by cropping (sensor size).


This said, you

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Originally Posted by Jon Ruyle

It may thus seem that crop cameras are better for extreme macro, but I don't find this to be the case: you can always crop a larger sensor, and usually it is diffraction rather than pixel density that limits resolution when you go way beyond 1x.

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Jon, this is how I have thought to. I would prefer the 5D II over the crop body.


tkerr's statement that the"1.6 x 100mm = 160mm, a 100mm lens on a crop body is equal to a 160mm lens on a FF Body", isn't entirely accurate. I know he was referring to crop size.


A 100mm on a 1.6 will have a DOF greater by almost 1.6 than a 160mm would have on a full frame body at the same apertures.


Lately thoughDenise has been posting macros that appear to have a fairly deep DOF, I believe using her new 70mm.


It makes me wonder if there is a benefit to shorter focal lengths using crop bodies for macro, just to get the deeper DOF at equal crop sizes.


Otherwise to the original OP's questions, it sounds like so far the consensus is that the crop does not affect the mm.

No. (Lateral) magnification is image height divided by object height.


Loosely speaking, the size of an object in the scene as projected by the lens, divided by the actual size of the object, is the magnification. So at 1:1 magnification, a 1 x 1 cm square object will project an image that is 1 x 1 cm on the sensor. It has nothing to do with the size of the sensor--even if the sensor is not big enough to fit the image, the magnification is still the same.


A 1:2 magnification ratio means the projected image has half the size of the actual object.


Note that this definition applies only to objects in the plane of sharpest focus.


It's not necessary to introduce crop factor or any other measurements when discussing magnification--like focal length, it is strictly a property of the lens.


Enlargement ratio, on the other hand, is defined as the size of the displayed image divided by the size of the sensor. So a 24 x 36 cm print (approx 9.5 x 14") taken from a full frame sensor would correspond to an enlargement ratio of exactly 10:1. Contrary to magnification, this is strictly a NON-property of the lens. The two are completely exclusive concepts.


Technical note. Because the real image formed by a typical camera lens is inverted relative to the object, lateral magnification is actually usually a negative number (the image and object heights have opposing signs). But for ease of understanding, the sign is ignored among those who are not optical engineers. However, in certain unusual cases, the sign cannot be ignored because the lens does not invert the image--with rather unexpected consequences.

Sensor size doesn

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Originally Posted by HDNitehawk

A 100mm on a 1.6 will have a DOF greater by almost 1.6 than a 160mm would have on a full frame body at the same apertures.

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By "at the same apertures" I think you mean "at the same f numbers". At the same apertures (ie, compare the 160mm lens at f/16 on FF with the 100mm at f/10 on 1.6FOVCF), the DOF would be exactly the same.

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Originally Posted by HDNitehawk

Lately thoughDenise has been posting macros that appear to have a fairly deep DOF, I believe using her new 70mm.


It makes me wonder if there is a benefit to shorter focal lengths using crop bodies for macro, just to get the deeper DOF at equal crop sizes.

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I think this effect is very slight. From what I've seen and remember, Denise tends to shoot excellent medium closeups at less than 1x. For these, one can get good DOF without too much diffraction just by stopping down (if you're reading, please correct me if I'm wrong, Denise).

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Originally Posted by neuroanatomist

At a given subject distance (assuming two lenses have similar MFDs for 1:1), the shorter focal length will result in deeper DoF. However, that's offset by the crop factor. The common wisdom that a crop sensor has deeper DoF is based on subject distance - to get the same framing, you're further away with a crop sensor, and the increased distance is what yields the deeper DoF. But at 1:1 (or for any fixed mag), subject distance is the same for APS-C vs. FF (the FoV is just smaller with crop). At a fixed distance, a crop sensor actually produces a *shallower* DoF for a given focal length.

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John


There are several ways to look at it, all of which are correct. I am not sure that any senario would give an advantage.


I think the bettercontrast out of the 5D IItrumps the crop bodies for macro.Thatmay not be true with the 1D IV, sometimes it seems to give better contrast than I get out of the 5D II. I may try it on my next outing for macro. Who knows,with its superior AF system I may be able to shoot some Bee's in Flight like Denisehas lately.

Thanks, that was the answer I was looking for, and I see how it makes sense. As for the DOF advantage, crop bodies would have it, but you can also stop down full frame cameras farther before diffraction starts to show..

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Originally Posted by mmodica

As for the DOF advantage, crop bodies would have it...

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Not true, as I stated above (unless you think shallower DoF is an advantage for macro shooting). Let's take an example - EF 100mm f/2.8L Macro IS on crop vs. FF. At 1:1 magnification, you're at the MFD regardless of sensor size, so that's 30 cm (11.8") from the sensor. On the 7D at f/8, your DoF would be 1.8 mm, while on the 5DII at f/8, your DoF would be 2.9 mm (i.e. 1.6x deeper DoF with FF). Feel free to plug the numbers into DoF Master. Plus, as you state, you can stop down farther with FF before diffraction has an impact, plus you get a larger field of view. So, it's a triple advantage to FF for macro shooting.

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Originally Posted by neuroanatomist

At 1:1 magnification, you're at the MFD regardless of sensor size, so that's 30 cm (11.8") from the sensor. On the 7D at f/8, your DoF would be 1.8 mm, while on the 5DII at f/8, your DoF would be 2.9 mm (i.e. 1.6x deeper DoF with FF).

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But 1:1 on the 7D gives you a smaller field of view than 1:1 on the 5D. 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.

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Originally Posted by neuroanatomist

So, it's a triple advantage to FF for macro shooting.

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The way I see it, there is *no* advantage either way. Let me explain.


In macro shooting, you never seem to have enough DOF. You can stop down, but this leads to diffraction. 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).

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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.

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

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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).

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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.

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Originally Posted by neuroanatomist

So, you can't take the same picture

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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).

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Originally Posted by neuroanatomist

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.

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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."

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Originally Posted by Jon Ruyle

<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).

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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:


http://farm5.static.flickr.com/4075/...0261cf96_m.jpghttp://farm5.static.flickr.com/4106/...07061aa2_m.jpghttp://farm6.static.flickr.com/5246/...8449c305_m.jpghttp://farm6.static.flickr.com/5032/...868f04c3_m.jpg


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.

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Originally Posted by neuroanatomist

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

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

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Originally Posted by neuroanatomist

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.

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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?

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Originally Posted by Jon Ruyle

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?

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

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Originally Posted by Jon Ruyle

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

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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.

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?