Math Help, I think...

BradS said:

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).

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