I recently picked up an issue of one of the nations most popular Photography magazines, and was shocked at what I read.
'Focal Length: This describes the distance (in millimeters) between the front of the lens and the image formed on the sensor or film plane when it is in sharp focus at infinity, that is , the farthest possible distance.'
From what I remember of my school physics (and I don’t remember that much) it’s the distance between the central plane* of a lens and the crisp in focus image of a subject at infinity! (*The central plane here just refers to the line running down the center of the lens cutting it into two equal halves. )
Needless to say that such a simplified explanation is not going to satisfy any one.
Now in terms of our Photographic lenses:
The focal length of a lens is defined as the distance in mm from the optical center to the focal plain ie. the sensor or film if the subject (at infinity) is "in focus". Period!