How far away do you want to be? That determines your perspective. Then choose a lens that frames it the way you want without wasting a bunch of film area.
Originally Posted by David Lyga
If you try to figure it out with mathematics it is very complicated, because, as you know, angle of view changes as you focus closer. That effect is greater with bigger format cameras.
A head-shoulder picture with a 20x24" camera is taken under conditions where the viewing angle is about one-half that of the angle of view at infinity. A head-shoulders picture with a Minox is taken under conditions where the angle of view is nearly identical to that at infinity.