(optics) A measure of how strongly an optical system converges or diverges light, the inverse of optical power. In the simple case of a single convex lens, it is the distance along the optical axis from the principal plane of the lens to the lens's focal point (where, for instance, the film would be placed in a camera). For more complex or multi-element optical systems, focal length need not correspond to any obvious measurable physical distance.

plural of focal length

An adjusted value of the equivalent focal length, so computed as to equalize the positive and negative values of distortion over the entire field used in a camera.

plural of calibrated focal length