focal point

optics of a converging lens, labeled diagram

Optics of a converging lens.

The focal point is the convergence point, or sharpest point of focus, for an image; also known simply as the focus. For a positive (converging) lens or mirror, the focal points of the element are the points at which a ray, approaching the element parallel to the optical axis, crosses the optical axis on the opposite side. For a negative (diverging) lens, the focal points are the points at which the backward extension of the diverging ray intersects the optical axis.