I have just assembled a 600mm Nikon ED telephoto from separately acquired front and rear elements to which I am fitting a new Copal 3 shutter. The shutter comes with shutter speed stops and scale but no aperture scale. The problem is to determine the locations of the aperture stops (and mark them with tape, or whatever).
From pictures of the same shutter calibrated by Nikon and provided with the complete new lens as part of the 600-800-1200mm set, it is clear that the diameter of the opening is a function of focal length. For a given aperture setting (say, f/64 in my picture), f/64 is a stop more open for the 800mm than for the 600mm, and two stops more open for the 1200mm than for the 600mm. My light meter has no setting for focal length, and the exposure it gives for a given subject is the same for any lens of any focal length. So each individual lens must be calibrated in such a way that f/64 corresponds to shutter openings of varying diameter depending on the focal length.
I put two lenses with calibrated shutters, a 300 and 450, on my 8x10, set each at f/11, focused, and took a meter reading of the same part of the subject off the ground glass from underneath the dark cloth. Pretty near the same reading. But when I measured as best I could with a ruler the width of the shutter opening at f/11, I got 2.5mm for the 300 and 3.5mm for the 450.
After all these years, none of this dawned on me until this morning. Could someone help with some basic optics? And an equation or two? There’s also the practical matter of positioning the shutter stops on my blank Copal 3. I suppose I could work with the light meter, opening and closing the shutter of the 600mm until I get the same exposure reading that I get on the 300 or 450 at a known aperture, then correcting for focal length. But for that I need a formula for making the correction. Thanks in advance for any help in clearing up my confusion.