focal length . A general term for the distance from the principal point or center of a lens to the principal focus. In photogrammetry, the term "equivalent focal length" is the distance measured along the lens axis from the rear nodal point of the lens to the position of the focal plane that provides the best average definition in the aerial negative, and the term "calibrated focal length" is an adjusted value of the equivalent focal length computed to distribute the effect of lens distortion over the entire field of the negative. Symbol: f.