With a DSLR, or SLR if you still use film
, 35mm focal length lens is probably the shortest you can get away with before you started really noticing the wide-angle effect of things closer to the lens getting really exaggerated in terms of size. In the studio a 85mm-200mm works great from controlling depth of field and compressing the image.
A close up butt shot with a 20mm lens is not going to be flattering, even on a 6'-0", 100 lbs. runway model.
If you are talking large format camera then the lens focal lengths are different.