Distortion at equivalent lengths

[flanniganj]

So I had a thought today and a search through some forums didn't reveal an answer to exactly what I'm wondering. I'm speaking in generals, not with any specific lens or any specific body, but here's my question: If you use a lens on a crop camera, and then on a full frame body, will the barrel distortion be the same at equivalent lengths? For example, will the distortion at 20mm on the full frame be the same amount at 36mm (the 1.6x equivalent) for the crop sensor?

If not, what causes the difference in distortion amounts?

Thanks for any help. Just curious.

[Janco]

I don't think it will be equivalent. The distortion is far more evident in areas of the image that are far from the center. Since Full frame has a larger image area the distortion comes more into play. Apart from that I guess that every zoom lens has different characteristics at different zoom levels. I hope that helps, and I hope I'm right  Oh and 20mm on 1.6 crop would be 32mm on FF 

[neuroanatomist]

Janco is absolutely correct.  First, though, let me correct a misstatement - a 20mm lens on FF is still a 20mm lens on APS-C, you're just cropping away enough of the image circle to result in an angle of view equivalent to 36mm on FF; focal length is an intrinsic lens property, independent of sensor.

As for distortion, that's intrinsic, too, but like many image artifacts (vignetting, lateral chromatic aberration, etc.), it's worse at the periphery of the image circle, and that's the part that's cropped away when using an EF lens on APS-C.

You can see this yourself - head over to photozone.de and pick a lens (easier with a prime, e.g 35/2 or 50/1.4) they've tested on FF and APS-C, and compare values for distortion (or vignetting) - they're always lower on APS-C for the same lens.

[Janco]

Am I really wrong with 20mm APS-C (x1.6) being equivalent to 32mm FF? Now that Neuroanatomist is talking about 36mm on FF as well, I finally get confused.... but that's not the topic anyway.

[Mt Spokane Photography]

Janco is absolutely correct.  First, though, let me correct a misstatement - a 20mm lens on FF is still a 20mm lens on APS-C, you're just cropping away enough of the image circle to result in an angle of view equivalent to 36mm on FF; focal length is an intrinsic lens property, independent of sensor.

As for distortion, that's intrinsic, too, but like many image artifacts (vignetting, lateral chromatic aberration, etc.), it's worse at the periphery of the image circle, and that's the part that's cropped away when using an EF lens on APS-C.

You can see this yourself - head over to photozone.de and pick a lens (easier with a prime, e.g 35/2 or 50/1.4) they've tested on FF and APS-C, and compare values for distortion (or vignetting) - they're always lower on APS-C for the same lens.

True,  but for example,  the Sigma 50mm f/1.4 edges the Canon 50mm f/1.4 on crop bodies, but the Canon may be better on FF, so you cannot assume crop comparison results also apply to FF. That part that is cropped away can be better on one lens than another.

[neuroanatomist]

Am I really wrong with 20mm APS-C (x1.6) being equivalent to 32mm FF? Now that Neuroanatomist is talking about 36mm on FF as well, I finally get confused.... but that's not the topic anyway.

Nope - my mistake, sorry! 

[flanniganj]

Thanks for clearing this up. Indeed, 36 was a typo, my mistake. Thanks again!