I'm not very good with mathematical formula, nor am I very good with the physics of lenses. I'm trying to figure out what size to make Water-house stops for Imagons.
If I'm not totally off-base, which could easily be the case, the relevant formula is:
Focal length/relative aperture = diameter of entrance pupil of lens, noting the that diameter of the Water-house stop does not equal the diameter of the entrance pupil.
One issue is that Imagons don't have optics in front of the stop. Does exit pupil diameter tend to equal entrance pupil diameter? That's more of a general interest question. I didn't figure this out to make my stops.
In looking at my 300mm Imagon, I noticed that a hole size of 31mm gives an "H" value of 7.7.
Because I'm an idiot, I plugged those values into the formula to give an effective focal length of FL = 7.7 x 31 = 238.7, thinking that I could use the 238.7 figure with actual hole diameters to give f-stop equivalents, hoping this took care of the entrance pupil/actual stop diameter situation. In practice, this worked fine....but I usually expose negatives a bit exuberantly, i.e. when in doubt give more exposure. So my method is good enough for me, but some people have inquired about me making stops for them, and I'd like to make sure I'm not leading them astray as to the f-stop equivalents.
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