30/05/2020
Without getting into an in-depth physics discussion, the focal length of a lens is an optical property of the lens. It measures the distance, in millimetres, between the optical centre of the lens and the camera’s sensor (or film plane). It is determined with the camera focused to infinity. Lenses are named by their focal length, and you can find this information on the barrel of the lens. For example, a 50 mm lens has a focal length of 50 mm.
In the definition of focal length, I mentioned the “optical centre” of a lens. You may be wondering what this is. Well, a camera’s lens is not made from a single piece of glass. Instead, it is a combination of lens elements and groups of elements. These combinations help to focus the light and cut down on distortions. The location where all the light rays converge to form a sharp image is known as the optical centre of the lens.
Focal length is a property of the lens itself, not the camera. What I mean by this is that a 50 mm lens is a 50 mm lens, regardless of whether on a full frame, cropped sensor, or medium format camera. However, the size of the sensor does play a role in the lens/camera combination’s field of view, but more about this in a minute.
