Re: [Leica] correction for previous post on lens design

["Ken Iisaka" <kiisaka@ibm.net>]

> I wrote:
> "On a very broad perspective one can argue that all lenses (except
> the zoom lenses) are all based on one design, the triplet (the Planar
> is a triplet too! 3 lens groups,identical to the Taylor layout)"
> Here I was too quick.
> The more complete reasoning is this.
> There are triplets with a 2-2-1 configuration. Adding one lens in
> front  makes it a 1-2-2-1 version, This is the Planar, which of
> course has four groups. Not three as I noted by a wrong count. It is
> 30 degrees C now.Not an excuse of course.

Well, it is not as crazy as it seems.

Cooke's triplet (1893), in positive-negative-positive configuration is a
foundation of symmetric configuration, with a coverage of 60 degrees (or so)
or less.  You could say that it is a simplification of Petzval (1840) though
the triplet configuration was among the first anastigmat, and was certainly
the simplet.

The Planar, or also known as double-Gauss configuration, is (+, +/-
(cemented), -/+, +).  The two sets of outermost positive element groups can
be considered equivalent of the positive elements in triplet, and the
cemented groups, both of which are, as a sum of two cemented elements,
constitute the middle negative element.

Similarly, Zeiss's Tessar, with (+, -, -/+), can be considered derivative of
triplet, although it is a much argued topic that has been beaten to death
many times as someone already pointed out.  Zeiss's Sonnar, which comes in
various flavours such as (+, -/+/-, +) and (+, +/-, +) clearly resemble the
basic power configuration of a triplet.

So, the triplet evolved into various configurations which are more complex.
It is interesting to note that the reverse of triplet, in
negative-positive-negative, seen in wide-angle lenses, almost started out
complex, then simplified.

It was not until 1950 or so until most wide-angle lens designs were still
derivative of the triplet configuration, with Hypergon (two positive
menisci) as one of the few exceptions.  Leitz's Summaron 28/5.6 is in
double-Gauss configuration, certainly stretching the limit of the
configuration.  Aviogon introduced in1950 or thereabout, with two sets of
two negative elements, with two sets of cemented negative/positive elements,
was certainly complex.  The subsequent Biogon, losing one of the outer
negative element, but adding another cemented element, was still quite
complex.  It was not until 1970 or so when Voigtlander, then owned by Zeiss,
introduced the infamous Hologon, the simplest superwide in existence, with
merely three elements in -, +, - configuration, inheriting the same power
configuration as Aviogon and orthometar.

Indeed, Erwin, a great deal of lenses are based on two simple designs:
triplet, and Hologon.  There are two other popular configurations though:
telephoto and inverse-telephoto, AKA Retrofocus.  Essentially, the former
has a (+, -) and the latter a (-, +)  Well, apart from mirror and zoom
lenses, the great majority of lenses are derived from these four
configurations.