Math Help, I think...

[Dan Quan]

I have 400mm rail plus 87mm flange and a old 360mm Ronar lens which needs 340mm for infinity. Is there a way to determine the distance from lens front to subject at max draw? I understand the Ronar is most bestest at around 2 meters or less, but that conversation is not the point of this post. I just need to know if I should get another 200mm of rail.

The bellows is not sewn yet so I cannot test.

Any help from the smart people would be greatly appreciated.

[Dan Quan]

D'oh, Thanks again Brad!

Dan,

the general equation is

1/f = 1/w + 1/v

where:
f is the focal length
w is the distance from lens to object (roughly what you call working distance)
v is the distance from lens to film plane (image distance)

so, for example,
if your focal length is 305mm and your bellows is 700mm (~27 inches) then

1/f = 1/w + 1/v
1/305 = 1/w + 1/700
1/w = 1/305 - 1/700
and so,
w = 1/( (1/305) - (1/700) )
and when I punch this into my trusty pocket calculator, I get...
working distance = 540.506 mm (~21.3 inches)

(EDIT: the two distances v and w are actually from the nodal point of the lens. This can be assumed to be roughly at the lens board for lenses that are not telephoto nor wide angle).