From the mailbag:
I want to buy a lens in the future for portraits primarily for self portraits. Most people want portrait lenses to produce pleasant images instead of realistic ones. For this, with my facial features favor a longer focal length. But it got me wondering how far away we have to be away from the subject for the subject’s face to look “normal”. Is there any good research on this? People have an image of what a person looks like in their head (presumably?), so how far do I have to be to replicate that?
The reason why I have such a question is because sometimes I want to take pictures to make things look nice (art) but sometimes I want to document reality (high resolution reality measurement device). But with portraits it’s very confusing to me how to shoot “accurate” portraits.
To reproduce the taking perspective in the viewing environment, it is sufficient to make the viewing distance the same as the capture distance. By viewing distance, I mean the distance from the viewer to the photograph. By capture distance, I mean the distance from the camera to the subject. So If you photograph a person from 1.5 meters away, if you view the image from that distance you’ll see a “natural” perspective.
Once you know that, the selection of the lens becomes a matter of knowing the viewing distance and picking the focal length that gives the desired field of view.
You may have noticed that I said the above is a sufficient condition. It is possible to relax the constraints somewhat. Note the above formulation means that the subject is life sized in the photograph. But you’d have the same geometry if you made the subject half sized in the photograph and viewed it from half the subject distance. However, in that situation, higher order psychological effects may come into play.