Originally Posted by
BetterSense
My brain sorta-remembers this formula from college physics. S_1 and S_2 are the distances from the rear nodal point to the film and the from the front nodal point to the subject, and I'm not sure what that means. If I do the calculation to find S_1 with f=90 and S_2 = 12 feet then I get 92.3mm. I take this to mean I would need to move the lens 2.3 mm closer to the film to get infinity, but I don't know about the nodal point thing.
How to find the nodal points is described here. But keep in mind the nominal focal lenght, e. g. 90mm isn't always the effective focal lenght. With the Super-Angulon 90mm f/8 it's 90.7mm +/- 1 %. This datas one can find normaly at the lensmaker's website.
For LF practice it's easier to calculate with the distance between the front of the lensboard and the film surface focussed at infinity. For the SA 90mm it's 99.4 mm. Now you can calculate sufficient all you need as a photographer. Like
magnification = focal length / (subject distance - focal lenght)
or
close-up exposure time = (magnification + 1)^2 * time of distant exposure
and many more
Peter
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