A lens gathers the rays of light coming from a scene and projects them as reversed images onto the film at the back of the camera. You can see this demonstrated below:

Many cameras use interchangeable lenses. The difference in types comes from their focal length - that is, the distance between the optical center of the lens and the point where it focuses (plane of focus). The focal length determines how close or how far the scene appears to be on the when it is projected on the film, which is called magnification. It also determines the amount of the scene in view, this is called angle of view. All these play together to determine how the foreground and the background appear to the viewer.

When a photographer speaks of the speed of his or her lens, they are simply refering to the measure of light that transmits through the lens to the film. A larger aperature lens allows more light than a small aperature lens. This diameter is measured as an f-stop. Faster lenses are often more expensive than slower lenses of the same focal length.

Above are the three basic types of lenses used in modern photography. The normal lens projects an image that would not vary greatly from what the photographer would see with his or her own eyes without looking through the camera. The telephoto on the otherhand allows the photographer to capture light from greater distances and make it appear to be closer than the scene really is. Finally the wide-angle does the opposite of the telephoto lens, it make the scene seem farther away and expands the viewable area.

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