Originally Posted by
aduncanson
This formula does not work too well for smaller reproduction ratios. It looks like it has a error of 10% at 3:1 and 100% at 1:1.
I would suggest m = f/(d-f) where d is the distance from the lens to the negative.
The only problem is that the effective location of the lens (its rear nodal point) is unknown for any practical enlarging lens. If precision is required then set up the enlarger to project a negative at 1:1 on the baseboard (some trial & error will be required) and measure down two focal lengths from the negative and make a mark on some part of the enlarger that is rigidly attached to the lens. That will be the effective location of the lens for measuring the "d" needed to calculate an unknown magnification in the future.
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