In another thread of mine (related to the benefits of APS-C vs full frame), the subject of "lens compression" came up, suggesting that if a 56mm lens is used on an APS-C camera versus an 85mm lens on full frame, the 85mm full frame setup might produce more subject compression (a desirable flattening of features in portaiture, for example). Some of us know this to be incorrect... we know that "lens compression" is a fallacy; but it's a common misunderstanding that I myself held when I was less experienced, until I was educated by more knowledgeable members here.
For the benefit of anyone still confused by this issue, I've carried out a quick test to prove that focal length does not affect subject compression ("perspective distortion")...
This image was shot handheld with my K-3 and Tamron 28-75mm f/2.8 lens, using a focal length of
75mm:
This next image was shot from the same distance, using the same camera and lens, at a focal length of
28mm:
Here's a
crop of the
28mm image:
Note that the
subject compression is the same. However, since both the 75mm and 28mm images were shot at the same f/3.2 aperture, the
depth-of-field is much shallower on the 75mm shot (as we would expect).
Here's a side-by-side comparison of the 75mm shot (left) versus the cropped 28mm shot (right), as it's easier to view this way:
Conclusion -
focal length does not affect subject compression. It's the act of moving closer to, or further away from, the subject that reduces or increases compression respectively. No matter what format of sensor and focal length of lens is used, if you shoot your subject from
the same distance, the perspective distortion will be the same.
Shooting a portrait from 5 metres using either a full frame camera and 85mm lens, or an APS-C (1.5x crop) camera and a 56mm lens (or thereabouts), will give you the same subject compression and a very similar field of view... but, depending on the maximum apertures of the lenses, the 85mm on full frame will likely be able to achieve shallower depth of field if required.
Last edited by BigMackCam; 12-03-2020 at 05:09 PM.