David, are you sure its back focus and not focal length? Really, really sure?
If you are, please direct me to a reference.
Dan, I just made an experiment with my "office ornament" Speed Graphic and a 240 mm Tele-Arton:
Camera extension at infinity focus 6.5", camera extension at nearly life-size (1:1) 12.5 inches (no more available on camera). Certainly no question of needing twice the focal length (480 mm extension) to get to 1:1.
Ummm... Why don't you look at the chart on the side of the bellows? C220 gives bellows exposure factor on a line at the bottom.
Poisonally, I would buy a rb67 with a 90mm lens and a #2 extender tube. Throw the dang paramender away.
tim in san jose
Exposure factor = (M/P + 1) ²
M = Magnification
P = Pupillary magnification
P is calculated as rear exit pupil diameter divided by front entrance pupil diameter (so it is specific to a particular lens).
Will send PDF file of these pages to anyone interested.
Oh. Correction for pupillary magnification. Well-known, been around forever, but often ignored.
When I bought my PB-4 in 1970, a sheet giving magnification vs. exposure factor curves for many, many Nikkors was packed with it. The curves for w/a lenses, all retrofocus, are terrifying.
IIRC, there's a full discussion, with formulae for lens facing normally and lens reversed in Lefkowitz.