Originally Posted by
neil poulsen
My 355mm G-Claron was originally barrel mounted. Purchased from Kerry Thalmann, he mounted the front and rear elements into a Copal 3 intended for a 210mm lens. I believe this means that, to obtain a given f-stop, I set the aperture to that f-stop on my 355mm G-Claron lens, and then open the aperture by a correction of 1.5 stops. (Actually, 1.52 stops.)
The calculation for this correction in stops isn't obvious and involves logarithms. The point is, one doesn't necessarily need to have a new aperture scale made, if the shutter you purchased was intended for a different focal length lens.
My calculation is the following . . .
Correction in Stops = log[355*355/(210*210]/log(2) = 1.52
This works for logarithms of any base. So, it works for log10(x), ln(x), etc.
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