Focal length in photography is measured in millimeters and is determined by the distance between the optical center of the lens and the image sensor when the lens is focused at infinity. It is a crucial factor in photography as it directly affects the field of view and magnification of the image. The focal length of a lens is typically printed on the lens barrel and can range from wide-angle (short focal length) to telephoto (long focal length). It is important to note that the focal length indicated on a lens is its true focal length only when used with a full-frame camera. When using a camera with a crop sensor, the effective focal length will be different due to the crop factor.