Maximum Magnification - different for EF lenses on crop bodies?

[mmodica]

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

[tkerr]

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.

[Jon Ruyle]

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

[HDNitehawk]

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.

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.

[wickerprints]

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.

[neuroanatomist]

Sensor size doesn

[Jon Ruyle]

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.

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.

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.

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).

[HDNitehawk]

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.

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.

[mmodica]

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..

[neuroanatomist]

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

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.