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focallength是什么

In optics, the distance from the middle of the lens to its focal point. Light rays run in parallel to one another, while lenses distort light rays: a convex lens (the type that is thicker in the centre than at the edges) brings them closer together and a concave lens (the type that is thinner in the centre) forces them apart. Cameras use convex lenses to bring the light rays together to a point where they converge and the image is in focus: this is where the film or light sensitive diode goes. The more convex (or thicker) the lens, the more severely the light rays are bent and the shorter the focal length (conversely, the thinner the lens the longer the focal length). Different focal lengths create different kinds of image effects. Lenses with very short focal lengths (or wide angle lenses) allow more of the picture to be seen and emphasize foreground elements, whereas lenses with very long focal lengths (or telephoto lenses) allow less of the picture to be seen and emphasize background elements which appear to be magnified. These different effects are dramatically illustrated by a technique used in feature films called a Hitchcock zoom, where the camera tracks out at the same time as it zooms in (or vice versa), an effect used in the film Vertigo (1958).

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