Online Tutorials: Focal Length
One of the most significant properties of any lens is its focal length.
The focal length of a lens is the distance that it should be held from a screen in order to project a focused image on that screen. This is true if your camera lens is made up of a single piece of glass, however your lens will often be made up of a number of pieces of glass (or elements), so a 300mm lens does not necessarily have to be 300mm long.
The focal length is key to the angle of view obtained. The longer the focal length, the narrower the angle of view and the more magnified the image is. At longer focal lengths the depth of field is reduced thus making focusing more difficult.
The next illustration represents the focal lengths and associated angle of sight for some standard lens focal lengths.