The "field of view" calculator ets 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 will I be able to fit 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 do I have to be to take a picture of a subject of a certain size?
To calculate the field of view, we first have to calculate the magnification of the lens, given its focal length and the distance to the subject:
where f is the focal length of the lens, and s is the distance from the optical center of the lens to the subject. (Roughly equivalent to the camera-to-subject distance, except for closeups.) Both values must be in the same unit, such as millimeters.
Given the magnification, the horizontal and vertical field of view are calculated:
where Fw is width of the film frame, Fh is height of the film frame and M is the subject magnification.