Definition - What does Focal Length mean?
In optics and photography, the focal length is the distance from the optical center of the lens to the imaging sensor when the lens if focused at infinity. The focal length is measured in millimeters. Focal length directly affects the size of the image captured, as it changes the angle of view. Wider angle view and greater area are captured when the focal length is shorter.
Techopedia explains Focal Length
In most cases in photography, lower optical power or longer focal length provides higher magnification. The angle of view in this case would be narrower. Conversely, higher optical power or short focal length is associated with lower magnification and wider angle of view. Focal length of the lens used in photography has many determining factors such as the radii of curvature, the medium in which the lens resides and the refraction index of the glass used for making the lens.
There are two main types of lenses used, namely prime lenses and zoom lenses. Prime lenses have a fixed focal length, whereas zoom lenses have variable focal lengths. The minimum and maximum focal lengths are provided in the case of zoom lenses. Lenses with focal lengths greater than fifty millimeters are called telephotos.
Variation in focal length allows the photographer to vary the distance between the camera and an object. It thus has an indirect effect on the perspective. In some cases, where focal lengths are smaller than the surface of the 35 millimeter film, focal length multipliers are provided for conversion to the 35 millimeter equivalents. In the case of digital cameras, optical zoom is calculated as maximum focal length/minimum focal length.