To DeadPoet

this is the older Summicron, not the new APO version?

from the Leica lens book

Quote

The first Summicron-R 1:2/90mm has been introduced in 1970
and has held the flag for this classical focal length during 30
years. It is a five element design from the Midland optical construction
department. It has some reminiscence to the Sonnar
design. The system is nose heavy as most elements are in front
of the aperture. The performance is good, but the Summilux-R
is as good and has twice the speed to illuminate the negative
area.

The Summicron-R 1:2/90mm displays the typical veil of soft
flare over the whole image at full aperture, that is the characteristic
of almost every high speed lens of the older generation.
The typical use of the 90mm as a portrait lens could serve as
an explanation and defense of this behavior. The soft reproduction
of image details and subject outlines created a more
friendly image of most faces of the sitters. But one did the
90mm lens an injustice as it restricted its potential universal
use.
The current Apo-Summicron-R 1:2/90mm ASPH (2002) raised
the level of optical quality substantially. The size is very compact
and almost equal to that of the current Summilux-R
1:1.4/50mm. Based on size it can be classified as a standard
lens. The optical quality is superb and a revelation. The aspherical
surface is grinded by CNC-equipment in a lengthy and elaborate
way. Grinding and finishing takes many hours per element
and is so time consuming because of the many
inspections and checks during the manufacture. If a lens is
manufactured with such accuracy, you need a very careful
assembly. The Leica-typical construction with metal parts of
exact dimensionality is a necessary condition for a high quality
product. This aspherical surface with a large diameter and complex
shape has to be assembled with utmost care to ensure
that the theoretical capabilities can be practically available.
The optical construction shows a five single elements, and the
first surface of the third lens has the aspherical shape. True
mastery can be seen in this elegant design. Some time ago one
needed seven or eight lens elements and could not get this high
performance. The low number of elements, the choice of glasses
for transparency and color transmission, the effective type
of coating, all work together to provide the remarkably clear
and crisp rendition. The image quality is very, even at full aperture
and veiling glare and secondary reflections are absent. If
you take pictures with the sun, directly shining into the front
lens, some aperture reflections can not be avoided.