f/Calc is a calculator for photographic formulas. These formulas
are at the heart of many interesting questions about the technical
side of photography. You could compute these functions by hand, but
they range from tedious to difficult, even with a good scientific
calculator. When you let f/Calc do the hard work, you gain an
intuitive feel for what the results mean, because you can quickly
try new input values to see how they affect the result.

WWW: http://tangentsoft.net/fcalc/