Quote Originally Posted by TheFlyingCamera View Post
Bellows compensation is pretty straightforward. For whatever lens you're using, no compensation is required when the lens is focused at infinity. At life size magnification (1:1), the bellows is extended 2x the focal length, and requires 2 stops. At 1/2 life size, the bellows is 1.5x the focal length at infinity, and requires 1 stop. From there you can calculate your intermediate compensation factors. Below 1/4 life size (a half-stop) unless you're shooting chromes, I wouldn't worry about exposure compensation, because you can't set less than a half-stop on the shutter speed anyway. With the Mamiya C2xx/3xx lenses, I don't know if the fstops are marked in less than 1/2 stop increments, but they're small enough it would be hard to precisely set a quarter-stop increment.
The only practical aspect to note is that exposure compensation is in fact dependent on the number of multiples of back focus distance the lens is extended, not focal length as such. From what I remember of my Mamiya TLRs, all the lenses up to 80 mm were retrofocus designs (back focus longer than focal length), the 105 was of conventional design, and all the longer lenses (135, 180, 250) were telephotos (back focus shorter than focal length).