| 1. | the distance from a focal point of a lens or mirror to the corresponding principal plane. Symbol: f |
| 2. | the distance between an object lens and its corresponding focal plane in a telescope. |

| focal length n. Abbr. f The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus. |
focal length n.
The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus.