The “field of view” calculator lets you solve two photographic problems. The primary one is, given a film size, lens length and distance to the subject, how much of the subject fits in the frame? By fiddling with the calculator a bit, you can also solve a slightly different problem: given a film size and lens length, how far away does one have to be to take a picture of a subject of a certain size?
To calculate the field of view, ƒ/Calc first needs to find m with Equation 7.1, “lens magnification”. Then it runs through the formula above three times with the horizontal, vertical, and diagonal dimensions of the imaging area for d. ƒ/Calc only has the horizontal and vertical dimensions of the imaging area in its frame size table; it calculates the diagonal with the Pythagorean theorem.