The 180mm f/3.4 Apo-Telyt-R reached the market in 1975. At that time it was no longer a secret that Leica had developed the lens for the US Navy as part of a high resolution 35mm camera system.
The system used Leicaflex SL2 cameras, the standard 35mm Summicron-R, a 75mm f/2 Elcan-R, a 180mm f/3.4 Elcan-R (later reincarnated as the Apo-Telyt-R), and a 450mm f/5.6 Elcan-R. This system was used by the US Navy starting in the early 1970s.
One of the problems of lens design is accurate color correction and the Navy presented Leica with the problems of developing lenses that could focus more than just the visible spectrum accurately.
Even today, most of the photographic lenses have what is referred to as “primary color correction,” where only part of the visible spectrum is focused at any time.
The solution to the problem lay in the development of glasses with what are considered “anomalous” properties; the combination of high refracting indexes with low dispersion.
Such properties are found in artificially grown crystals; an example is the calcium fluoride elements made famous in the Canon FL series telephoto lenses.
But such crystals have a very large temperature coefficient, and elements made from them are both brittle and extremely soft. The temperature related expansion of calcium fluoride elements is so great that most lenses made with them are subject to changes in focal length with temperature changes, and therefore have no proper infinity stop or distance markings.
The softness of the material also leads to design constraints. For instance, the Canon 300mm FL lens has a thin, conventional glass element in front of the “front” calcium fluoride element, primarily for protective purposes.
Not an ideal situation. Lenses made of these crystal elements demand extreme care to assure proper performance, and the military considered them incompatible with the kind of treatment they were likely to receive.
The glass research lab in Wetzlar set out to develop a glass that had the optical properties of crystals like calcium fluoride, but without the negative side effects. They did indeed develop such a glass, today commonly referred to as “apo glass.” It was/is used in a variety of Leica lenses, including the Apo-Telyt, the 800mm Telyt-R, and the Noctilux 50mm f/1. Their designer, Dr. Walter Mandler, was the man whose genius brought us those lenses, but also the 35mm Summilux, and close to fifty other lenses for Leica cameras, in addition to lenses for RCA television cameras, IMAX projectors, and Picker X-ray equipment.
How good are the Elcan lenses, specifically the Apo Telyt 180mm f/3.4? While most photographic lenses have a color correction from 400 to a maximum of 700 nanometers, the Leitz apo glass allows correction up to 900 nanometers. In simple terms, this means that all colors of the visible spectrum and infrared are focused in a single plane.
The Apo-Telyt proved to be the best lens of the set, making it one of the very few lenses for 35mm cameras that do not require refocusing when used with infrared films or sensors.
The US Navy conducted comparison tests with the 180mm f/2.8 Elmarit-R. These demonstrate the effects of the apochromatic correction of the Apo-Telyt. Both lenses were tested at f/3.4 with blue, yellow-green, red and infrared light. The maximum focus shift of the Apo-Telyt was +/- 0.045mm. The shift of the 180mm f/2.8 Elmarit-R (at f/3.40 was +/- 0.25mm.
In practical applications, such correction translated into fantastic sharpness. Increased resolution is readily apparent, made possible by greatly increased contrast capabilities.
The Navy test showed that the Kodak 5069 film, developed in H&W 4.5 developer consistently achieved resolution figures of 600 lines per millimeter. To make enlargements with this kind of detail required a specially modified Leitz Focomat II enlarger and lenses.
It is safe to say that regardless of manufacturer, the Leitz Apo-Telyt-R 180mm f/3.4 is still one of the very best lenses ever made for 35mm photography.
For more on the subject go to:
The Leica Camera Blog
Carl Merkin: The spy Who Came In From The Cold…The 180mm f/3.4 Apo-Telyt-R
MANUFACTURE AND PERFORMANCE OF PHOTOGRAPHIC LENSES
This article has a lot more information on the Apo-Telyt and the tests conducted by the US Navy.
In Memory of the LEITZ GLASS LABORATORY