When we use a real-world lens: multiple thick elements and cemented compound elements having different indexes of refraction, the lens simply wonít conform exactly to the thin lens equation. This is annoying insofar as the calculated values differ from the actual values produced by the lens. If our needs arenít fussy, the approximations the TLE provides are useful and reasonably accurate. The calculated values for typical enlarger lenses are somewhat less than the actual measured values. The enlarger lens behaves as though it had a somewhat longer focal length than specified by the maker.
By measuring the negative-to-print distance and magnification of the projection, we can calculate the focal length of the imaginary equivalent thin lens that corresponds to these measured values. By substituting the equivalent focal length into subsequent equations we can get better approximations to the actual values of our system.
Iíve found that for the 6-element enlarging lenses Iíve tested and for which I have the makerís specified focal length, the ratio (equivalent thin lens focal length)/(actual focal length) is about 1.026, i.e. the equivalent focal length exceeds the actual focal length by about 2.6%.