There’s rarely a need to do much calculation in the course of ordinary photography but occasionally it’s helpful to determine the effect of filter factors or estimate the effective depth of field for some subject distance. Although it’s not easy to work these answers in your head, they’re easy enough to find if you have a calculator. Here are a few equations that you may find useful.
The lens aperture calibrations, or f-numbers, express the ratio of lens focal length to aperture diameter. Both measurements must be in the same units. If A is the f-number (relative aperture), F is the focal length, and O is the aperture diameter, the equation is:
For example, if the focal length is 240 mm, and ...
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