- the distance from a focal point of a lens or mirror to the corresponding principal plane. Symbol: f
- the distance between an object lens and its corresponding focal plane in a telescope.
- the distance from the focal point of a lens or mirror to the reflecting surface of the mirror or the centre point of the lens
- The distance from the surface of a lens or mirror to its focal point.focal distance focus
- The distance between the optical center of a lens or mirror to its focal point.
The distance between the focalof a (that is, the point at which the lens will focus parallel rays of light) and the lens itself.