|Note that the angle of view produced by a given lens focal length depends on the format (size of the sensor) of the camera with which it’s used. A 50mm “normal” lens on a “full-frame” digital SLR provides a 46° angle of view (measured diagonally, corner to corner). A smaller APS-C sensor “sees” a smaller portion of the image produced by the lens, so the 50mm lens produces an angle of view of 31°30’, about what a 75mm lens would “see” on a full-frame or 35mm camera. On a Four Thirds System camera, with its even smaller sensor, the 50mm lens will produce a 23° angle of view, equivalent to what a 100mm lens sees on a full-frame camera.
This means a 35mm lens is “wide-angle” on a full-frame DSLR, but would be “normal” (52.5mm-equivalent) when used on an APS-C DSLR, and “portrait telephoto” (70mm-equivalent) when used on a Four Thirds System DSLR. In “telephoto” terms, a 200mm short tele on a full-frame DSLR frames like a 300mm tele on an APS-C camera, and like a 400mm super-tele on a Four Thirds System DSLR. You have to think in terms of angle of view for the format, or “equivalent focal length,” rather than just the raw millimeter count, when considering a lens for a specific project.
Some consider the effect of smaller sensors on angle of view to be a “magnification,” but this is not technically the case. A given focal length produces a given image size at the focal plane, and this doesn’t change because you change the size of the image sensor or film at the focal plane. If a 100mm lens focused at infinity produces an image of a specific distant tree a half-inch high at the image plane, it’ll be a half-inch high at the image plane whether there’s a 17.3x13.0mm Four Thirds sensor there or a 36x24mm full-frame sensor there. The magnification produced by the lens doesn’t change. However, that half-inch image will take up nearly the entire height of a horizontal Four Thirds System image, and just half the height of a horizontal full-frame image. So for practical purposes, you can consider the smaller sensor’s effect to be magnification, but technically, it isn’t.