Basic Features of Lenses

Focal length refers to the distance from the lens to the image sensor (CMOS sensor) when the picture is in focus. The focal length is listed on most lenses.
Field of view refers to the area that appears on the screen as an image. The shorter the focal length, the wider the field of view, and the longer the focal length, the narrower the field of view. For example, a 100mm lens has twice (2x) the focal length of a 50mm lens, so it will have a field of view that is 1/4 the size (1/2 x 1/2).