Lenses on Crop Sensor Cameras and Distortions

hanhasgotqi wrote:

First of all, a big thanks to DPReview for helping newbies like me learn the basics of photography.

I need to take some close-up shots of villagers where I live, for a research on facial deformities and diets. I'd like to avoid distortion of any kind when taking these photos. Ideally the faces in these photos would look very much like how we see them in real life. I'm referring to both the widths of faces and facial features.

I understand that, on a FF camera, a 28mm lens will distort my subject a little (close-up shots), and a 135 mm lens will flatten my subject’s features. My question is: what determines this distorting or flattening effect? Is it angle of view or focal length?

The angle of view of the object relative to the axis. Even with a very wide lens, faces close to the center will look normal.

In other words, if I use a crop sensor camera like Canon 70D, and a 28mm EF lens, will faces in close-up shots look slightly distorted, like what we expect a 28mm lens on a FF camera would do, or will the faces look quite normal, like the ones taken with a (hypothetical) 44.8mm EF lens on a FF camera? (I'm under the impression that 40mm-65mm lenses are considered normal lenses on a FF camera - not distorting subjects)

Yes to 44.8mm. BTW, there are distortions at any FL, just less visible. They are not really distortions in the sense that the lens may do a perfect rectilinear projection but the faces will still look distorted to us.

If you want to minimize those distortions, choose longer FL's or do not allow people near the borders. Another option is to use software correcting "volume anamorphic distortions" like DXO's ViewPoint. See this, for example. I use it often and the effect is very nice.