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 perspective, which is associated with the angle of view.

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)

Please correct me if I've got any point wrong. Thank you!

Perspective doesn't change with the crop factor, so a 50mm lens still looks like a 50mm lens, it's just that you get a crop of that. So I would choose the best focal length that works for your project, in this case probably an 85mm, just take into account its minimum focusing distance which is about a meter if I recall correctly. A prime would have the advantage that you wouldn't run the risk of taking different subjects at different focal lengths like with a zoom, and be less cumbersome.