In both physics and photography, the focal length of a converging lens is defined as the distance from the thin lens to the point were collimated light striking the lens will converge into focus. Collimated light is defined as rays that are all originating from the same point that is at infinity from the lens and arriving at the front of the lens parallel to each other.

If the point from which the light rays are originating is closer than infinity, then the rays from that point will not strike the lens parallel to one another. Such rays will also not converge into focus at the lens' focal length, which is always defined based on where collimated light from a point source at infinity converges. In such a case, the distance between the lens and the imaging plane (film/sensor) will need to be greater than the lens' focal length. The closer to the lens the object that we want to focus upon is, the longer the distance between the lens and the image/film/sensor plane needs to be.

When a lens with multiple elements is used, focal length is measured from the point a theoretical single thin lens would be located having the same refractive properties as the combined multiple elements have. This point can be either in front of the front element of the lens (i.e. a telephoto lens), behind the rear element of the lens (i.e. retrofocus lens), or anywhere between those two points.