[Leica] lens repolishing

[Andras Iklody-Szabo <isza@link7.lat.net>]

On various occasions there were mentions on this list of scratched lenses
which could be restored to as new condition, but this was not considered an
economically feasible solution.

Let's forget about the cost for a moment! What I do not understand with
this procedure is, how can a lens be restored to as new condition
photographically (not cosmetically), when repolishing alters the lens'
geometry. Factory tolerances when grinding lenses are in the order of
.001mm or so. A scratch on the front or back surface of a used lens can
easily be of the same order of magnitude, if not more. To grind (polish) it
away, you have to take away that much glass from the whole lens surface
(not just locally). That means that the geometry (thickness) of the lens
and thus the path of the light rays through it are altered. Since the
damaged lens surface is usually up front, this alteration changes (if ever
so slightly) the path of the light rays through the folloing lenses in a
multielement lens as well.

How does all this affect photographic quality? Does it only translate into
a slight shift of focus for which the lens is then adjusted (e.g. the
polished element shimmed) or is there more to it? I am not an optical
expert but somebody on the list might be. Can he help?


Andras Iklody-Szabo
Caracas / Venezuela