| 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 distance n. Abbr. FD See focal length. |
| 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 distance n.
See focal length.
focal length n.
The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus.
| focal length (fō'kəl) Pronunciation Key
The distance between the optical center of a lens or mirror to its focal point. |