The one characteristic of a camera lens that affects the type of image more than anything else, is its focal length. The focal length is the distance from the lens to the focused image when the lens is set for infinity. In a camera, the image is focused at the film plane. A lens that has a short focal length, 28mm for example, bends the incoming light rays sharply in order to record a large part of the scene in front of the camera. A long focal length lens, on the other hand, refracts the light at a more gentle angle, magnifying the image, but taking in only a small area of the view in front of the camera.
Only the most basic 35mm cameras have a single, fixed focal length lens. The majority either have a permanently fitted zoom lens that covers a range of focal lengths, or the facility to change lenses. In any case, a choice of focal lengths is essential for dealing with a variety of subjects and being able to treat them in different ways. Although the simplest difference that focal length makes is the angle of view (taking in a broad or narrow part of the scene), there are more complex differences in the style of the image and the interpretation of reality.
Traditional lenses were designed to work at a single focal length, and it was this that made the concept of interchangeable lenses the mainstay of professional camera systems. Zoom lenses, more complex in design and manufacture, provide a range of focal lengths in one lens. In their early years, zoom lenses were bulky, slow (that is, with smaller maximum apertures than regular lenses) and optically inferior, but continuous improvements have now made them a true alternative to fixed focal length lenses. As a result, except for extreme wide-angle and telephoto photography, a couple of zoom lenses can take the place of an interchangeable-lens camera system. The advantages are convenience and simplicity, while the range of angle of view that they cover is continuous. The disadvantages are that the maximum aperture is never as wide as an equivalent fixed focal length lens and certain specialized fields of photography, such as wildlife, need extreme focal lengths.
