At 07:33 PM 7/30/03, Lama wrote (in part):
If FL= focal length in mm, and E is extension in mm, and FF is the f-stop
factor, here's the formula:
FF = (FL + E)/FL
Note:
This presumes focus setting on lens focus helical is at "infinity." If
it's focused closer than that, the lens is extended further by its internal
helical, in addition to the extension tube, and that extension must be
added as well. The equation I use is similar, but also takes into account
extension of the lens with its own focusing helical, FE:
FF = (FL + LE + E)/FL
-- John
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