Focal length is a property of a lens that controls the angle of view and magnification of a photograph. It is usually represented in millimeters (mm) and is not a measurement of the actual length of a lens, but a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor or 35mm film at the focal plane in the camera. The focal length of a lens is determined when the lens is focused at infinity.

The longer the focal length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification. Focal length is important because it relates to the field of view of a lens, which is how much of the scene will be captured, and it also explains how large or small a subject in your photo will appear.

There are two types of lenses: prime and zoom. Prime lenses have a fixed focal length, while zoom lenses have variable focal lengths. The best focal length for a photograph varies based on the subject matter and situation.

In summary, focal length is a fundamental concept in photography that determines the angle of view and magnification of a photograph. It is a property of a lens and is represented in millimeters. The longer the focal length, the narrower the angle of view and the higher the magnification, while the shorter the focal length, the wider the angle of view and the lower the magnification.