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?

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!

As others pointed out, around 50mm on FF (crop: 50/1.6) gives about the look the human eye is used to in terms of angle of view. However, portraits are usually taken with slight tele, i.e. 85, 100, or 135 (FF). As pointed out, they key to perspective is distance. A person imaged with a wide-angle lens at 10meter distance in the center of the frame will have a perfectly fine portrait, but you might be left with a 100x100pixel crop (or whatever) for the portrait.

the key about perspective: closer objects are larger than further distant objects. (i.e. the big nose with wide angle portraits). If the person is very far, the distance to the nose, and to the ears will be virtually the same in relation to the total distance. For scientific purposes, it's better to have little perspective distortion, because that means, you can take a ruler on the image, measure features of the face, and they will be to scale.

Thus, I would recommend to go as long as you can (space, good illumination) to get relatively "flat" images. 100mm, 200mm whatever is feasible. Try to use always the same distance (like mug shots), even with a ruler in the background, then you can compare different shots. Of course, you may then take specific close-ups of particular features in addition.