A convenient way to measure the focal length of a positive lens makes use of the following...

A convenient way to measure the focal length of a positive lens makes use of the following fact. If a pair of conjugate object and (real) image points (S and P) are separated by a distance L > 4f, there will be two locations of the lens, a distance d apart, for which the same pair of conjugates obtain. Show that

Note that this avoids measurements made specifically from the vertex, which are generally not easy to do.