Quote Originally Posted by Mateo View Post
this can be easy if you want it to be

if you got a 5.6 inch lens and you extend the bellows to 8 inches you need to add a stop
if you got a 5.6 inch lens and you extend the bellows to 11 inches you need to add 2 stops

if you got a 210mm lens and you extend 320mm you ain't gonna be far off if you add 1 stop
if you got a 210mm lens and you extend 450mm you ain't gonna be far off if you add 2 stops

see the pattern, forget about inverse square law this is quick and dirty and if you can be more accurate you have some super calibrated lenses and need to calm down
exactly!