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I would like to do some portraits with my 14x17 which has a max draw of 42". I am thinking about getting a 30" lens but conscerned that I may not have enough bellows draw. Is there a way to determine how much bellows I need for a lens of a specific focal length?
Depends if full body or just head
I have a life size plastic model that sure helps during setup
Use the thin lens equation.
(1/object distance)+(1/image distance)=(1/focal length)
1/O+1/42=1/30
Works out to O=105 so you can focus that 30" lens down to 105" (8'9") from the lensboard. That's a comfortable working distance. I wouldn't want to be any closer for a portrait.
M = (B-F) / F Gives magnification %.
M=2/5=40% so you can reproduce something just under half size on film. That seems about right for a bust on 14x17 just thinking about and not measuring my head.
You might want something more like a 19" if you want tight "head shot" portraits and be right about 1:1 with some wiggle room.