You can
put this solution on YOUR website!the relative aperture, or f-stop, of a 23.5-mm diameter lens is directly proportional to the focal length F of the lens.
f = kF
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If a 150-mm focal length has an f-stop of 6.3,
Find "k".
6.3 = k*150
k = 5.3/150 = 0.0353333...
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Equation: f = 0.035333*F
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find the f-stop of a 23.5-mm diameter lens with a focal length of 80 mm.
f = 0.03533*80
f = 2.83
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Cheers,
Stan H.