f/Calc is a small utility for photographers that calculates various lens formulæ. Currently, it understands how to calculate the following:
All you have to do to use it is tell the program a few things about the situation, like the focal length of the lens in question, and the film size you're using.
The program is also fairly intelligent. If you enter a value in one location, such as the distance to the subject, all parts of the program will use that new value, so that you can calculate several different things, without having to reenter these values in each part of the program. Also, the program recalculates the answers every time you change a field, so you can easily try many different values to see what effect changing the inputs has.