[Leica] Coatings and stuff

["Erwin Puts" <imxputs@knoware.nl>]

When evaluating lenses and/or discussing optical properties or develoments,
some background knowledge is most certainly necessary. Otherwise it might be
the case that attention is drawn to the wrong aspects or even
characteristics are mentioned as discriminative in a negative or positive
way do not  have the  relevance or importance attributed to it. The Rokkor
topic does bring home this observation with some force. The study of lens
drawings is a case in point. Without knowledge of optical design, without
knowing the optical specs of a particular lens, the comparison of two
diagrams is very dangerous and most certainly will lead to misleading
conclusions. Two identical diagrams can deliver significantly different
performance and two digarmas that look different, might give comparable
performance. The assumption that the diagram reflects optical quality is
erroneous. If an optical designer is presented with whatever lens diagram
and you would ask him/her to make any statement about performance or even
assess differences, he/she would politely note that this is impossible
without having access to much more important info. The noted difference of
the third lens element between the Rokkor-CLE and the Summicron/Rokkor-C is
obvious, assuming that the drawings are faithful. A Double Gauss lens is
however, remarkably insensitive to small changes in glass thickness and
without having any idea of the true radius of the glass and its type and the
tracing of the rays, any conclusion, however tentatively, about design
changes pointing to possible improvements, is a shot in the dark and because
not based on analysis, inherently misleading. The only statement that can be
corroborated is this: on the assumption that the diagramas are faithful, we
note a difference in thickness of the third element, the purpose of which is
unknown, without additional info.
Coating is a second topic that is easily mis-interpretated. Single coating
is an obvious technique, as is multiple layer coating: in the first case a
lens surface is coated with one layer of a certain and in the second
technique,  several layers are deposited, from two to nine and even more per
surface. ML-coating is not in itself better than SL-coating, it depends on
the design, on the glass types used and more. As example, when using high
refractive glass, a SL-coating is more efficient than a ML-coating. If the
Summicron were SL-coated, but used high RI glass, the effect might bettetr
than a Minolta lens with ML and low refractive glass. ML-coating is often
also used as a means to correct the colour transmission of a lens, again
depending on the glass used. Leitz used three layer coating on selected
surfaces of some lenses already in 1957, but did not mention it
specifically, as they gave this aspect no public relation relevance. So if
Leitz notes of a lens that it has coating, the inference that this has to
imply SL-coating  is incorrect and even  if a certain lens does have single
coating, that is not a sure sign of inferior performance.
The idea that MC-coating is more effective in flare reduction and repression
of secondary images as SL-coating, is not true either as a general
statement. And the claim of a Lugger that the Summicron-C must be a
SL-coated lens, as his Summicron-C does have a significantly higher flare
level than the Rokkor-CLE, is quite rash. Read what Mr Crawley, of BJP fame
noted about the Summicron-C: "the lens is flare-free at full aperture". If
the Summicron-C, as claimed, is of SL-coating type and as claimed, the
SL-type is of inherently higher flare level, such a remark were hard to
accept.
The type of coating is a lens characteristic that merits attention, but only
in the context of the rest of the design parameters and without this
knowledge, the singling out of coating properties as distinctive elements of
the relative performance of a lens, is more reminiscent to a marketing act
than enlighenment and advice for the user.
Erwin