Quote Originally Posted by MattKing View Post

I don't disagree with you that many of us do use our experience with the common "portrait" focal lengths for 35mm film when we consider this issue. The problem that occurs, however, is that trying to covert that experience for use with other formats can be both complex and subject to error.

I shoot several medium format formats, and in at least one case I have to add close-up accessories to my lenses to attain a close up headshot using an appropriate lens. In the end, for me it is simpler to arrive at a single, standard working distance for each intended result (full body portrait, 3/4 body portrait, upper torso and head portrait, shoulders and head portrait, close up headshot) and then choose the appropriate lens to match.
Matt, you're making things more difficult than they need to be. Simply understand the ratio of your [favorite 135 format FL:135 frame height], and perpetuate that ratio for all other formats, choosing the closest real FL lens!!!

You like 85mm at 9' distance for 135 format head and shoulders; 85:24 = 3.55:1. Use
  • 150mm (3.55 * 43) on 645
  • 200mm (3.55 * 56) on 6x6 or 6x7
  • 320mm (3.55 * 90) on 4x5"
  • 65mm (3.55 * 18) on half frame (e.g. Olympus Pen)

You like 150mm at 9' distance for 135 format headshot; 150:24 = 6.25:1. Use
  • 270mm (6.25 * 43) on 645
  • 350mm (6.25 * 56) on 6x6 or 6x7
  • 560mm (6.25 * 90) on 4x5"
  • 110mm (6.25 * 18) on half frame (e.g. Olympus Pen)

...results identical framing and identical perspective at the identical subject distance
choosing the closest real FL lens.