What is the focal length of a lens?

Focal length is measured in millimeters, but the measurement is not of the actual physical length of the lens, but rather the magnification properties of the lens. The actual measurement of focal length is the distance, in millimeters, between the convergence point and the imaging sensor. read more

For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens (for example a convex lens), the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot. read more

The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters (e.g., 28 mm, 50 mm, or 100 mm). In the case of zoom lenses, both the minimum and maximum focal lengths are stated, for example 18–55 mm. read more