How much bellow draw needed for 30" lens for 14xx17

[Michael Wellman]

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?

[Eric Leppanen]

http://kennethleegallery.com/html/tech/bellows.php

[Tin Can]

Depends if full body or just head

I have a life size plastic model that sure helps during setup

[C. D. Keth]

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.