The Blender camera can be used much like a film camera. A film camera has three basic controls: the lens, which controls what you see; the aperture, which controls how much light you see; and the shutter, which controls the length of the exposure. Using the aperture and the shutter together, you can control how bright the scene is and how much detail you can see.
The field of view is the area that can be seen through the camera. This can be thought of as the width of a cone in front of the camera, measured in degrees, as shown on the left side of the following screenshot.
Blender allows you to specify this angle in two ways, in degrees, or as the size of a camera lens that sees a scene of that width.
By default, Blender specifies the field of view as a lens of a certain Focal Length, as seen on the right of the following graphic. The focal length is specified in millimeters, in the same way as cameras do. This lets you work more...