Gilligan wrote:Thanks for the help guys. I promise I am not as ignorant as I came across in my post with such a goofy question. For some reason I thought that the focal length for a lens varied with sensor size, when in fact the focal length for a lens doesn't change if you put it on a camera with a smaller or larger sensor.
The way I like to describe it fairly straightforward. Imagine a pinhole camera: you have a box, with a light-sensitive plate or film of some sort at one end, and a tiny hole in the centre at the other end. The focal length of the lens is the distance that pinhole has to be from the plate to form the same image as the lens. Note that it doesn't matter how big the plate is; even if the lens only forms an image circle that covers only a part of the plate, it still has a focal length that matches the distance the pinhole needs to be to form the same image within that particular area.
Conversely, the field of view depends upon the distance the pinhole lies from the plate, as well as the size of the plate - basic trigonometry. If you cut the size of the plate in half in both dimensions, the area it captures from a given lens will be halved in both dimensions, giving a similar effect as if the focal length had been doubled - but the lens is still the same focal length; only the field of view has altered.
So a 50mm lens is "normal" on a standard 35mm film camera; a moderate telephoto on APS-C digital sensors; a longer telephoto for a compact camera (which generally has a tiny sensor); and a wide angle lens for a medium or large format camera. In all cases, the lens is still a 50mm focal length lens; it's the image sensor size that varies.
The major reason all this matters is depth of field, which is (to a first approximation) a function of the absolute aperture size: a 50mm f/2.0 lens has the same absolute aperture size as a 100mm f/4.0 lens, making it easier for a large format camera to get a shallow depth of field than a 35mm or APS-C camera (handwave, handwave.) Whether this is a good thing or a bad thing depends on what you're trying to achieve.