By Heinz Richter
In 1952 the Ernst Leitz company (the former name of Leica Camera) established an overseas factory in Midland, Ontario, Canada. The company was called Ernst Leitz Canada and produced a variery of items under the ELCAN (Ernst Leitz Canada) label. Leitz had always manufactured specialty items for the military and ELCAN was no exception. In the 1960s they were asked by the US Navy to develop a high resolution 35mm camera system. This was based on the Leicaflex SL.
Leicaflex SL with 75mm f/2 ELCAN Lens
Photo: James L. Lager (ed.), Leica Illustrated History, Vol. II, 1994, p. 307, 309 and Vol. III, 1998, p. 319
Resolution, the ability of a lens to project fine detail has always been a measure of great importance in the evaluation of photographic lenses. The following are excerpts from an article by Joseph A. Schantz, Assistant Head of Research and Development Department at the Navel Photographic Center in Washington, DC.
Research by optical glass manufacturers has resulted in optical glasses having improved properties with respect to secondary color aberration. Since this glass became available lenses having complete color correction over an extended spectral range have been developed for a Navy 35mm camera system.
Since 1963 the Navel Photographic Center and the Navel Air Systems Command as a matter of continuous policy have expanded efforts to upgrade 35mmphotography on a systematic basis. The aim of this work was not only to improve the quality of documentary and reported photography but also to improve intelligence collection capabilities of the Navy’s cameras.
While most of this type of photography by the Navy is classified, one application, according to a former Navy officer, was in the Mediterranean. For instance, US Navy ships often came into close contact with Russian Navy vessels. It was common practice to shoot a series of photographs of these ships during such encounters. The relative ease of operation of a 35mm camera, compared to medium format and even more so large format cameras, proved to render better photographs with 35mm equipment. It should be noted that the article by Mr. Schantz was written prior to the advent of digital photography. However, the principles of lens design are essentially the same in digital photography also. To get the necessary detail compared to medium and large format photography it was necessary to develop a special, high resolution 35mm camera system. Mr. Schantz further writes:
It was recognized that the resolution of a lens is limited in part due to a lack of correction of secondary colors. Based on the light refracting properties of available optical glasses, most lenses can only bring two wavelengths to a common focus. The optical designer must choose these wavelengths to best suit the intended use of the lens. All other wavelengths are then focused in planes predetermined by the properties of the glasses over which the designer has no control.
Until the advent of extra low dispersion glasses the only exceptions were apochromatic microscope lenses and some telephoto lenses for 35mm camera systems which incorporated lens elements made of calcium fluoride crystals or alum and some process lenses that used large opposite powers in their element configuration. But these were limited to apertures of f/9 or smaller.
Lenses utilizing elements made of calcium fluoride have proven to be of very good performance. But they are not without drawbacks. Calcium fluoride unfortunately is very soft and scratches easily. Therefore front elements made of calcium fluoride usually are protected by a clear front glass plate or a thin lens element made of conventional optical glass. Furthermore calcium fluoride has a rather high temperature coefficient which means that it expands and contracts very much during temperature changes. In some instances this had led to calcium fluoride lens elements literally shattering when objected to great temperature differences. It is also the case that these elements do change their optical properties during temperature changes which makes it necessary to allow lenses incorporating such elements to be focused past the infinity mark. With other words, while the performance of such lenses is quite high, this must be paid for with a number of drawbacks.
However, research has produced optical glasses that do display the same or similar properties as calcium fluoride, but without the above mentioned drawbacks. The first such glass was developed in the late 60s and early 70s by the glass research laboratory at Ernst Leitz Wetzlar, the maker of Leica cameras. Not until approximately ten years later were similar glasses made available by other glass manufacturers like Schott and Nikon. These glasses are commonly referred to as extra low dispersion or fluor crown glasses. They allow the easier design and manufacture of apochromatic lenses. Only true apochromats have the ability to focus all colors of the visible spectrum at one common point. Mr. Schantz further states:
A series of such apochromates has been designed and manufactured under contract by Ernst Leitz Canada Ltd (Elcan). The lenses were designed by Professor Dr. Walter Mandler. The lenses consist of: Elcan-R 75mm f/2, Elcan-R 180mm f/3.4, Elcan-R 450mm f/5.6.
ELCAN 75mm f/2
Photo: L Camera Forum
180mm f/3.4 Apo Telyt
Schematic of the 180mm f/3.4
Elcan-R 450mm f/5.6
These lenses were designed for the Leica R cameras and are color corrected from 400 to 900 nanometers. These lenses permit photography in black and white, color and infrared with the same focal setting. When subjected to a series of filtered responses,(measurements with colored light) of blue, yellow, red and infrared, the yellow, red and infrared responses were grouped over a focal plane spread of only 0.02mm. (These are data for the 180mm f/3.4 which was tested at maximum aperture. Initially a classified piece of equipment, the lens later became available on the civilian market as the 180mm f/3.4 Apo Telyt-R) The curve of the blue filter is shifted 0.07mm closer to the lens. The maximum focal plane shift from blue to infrared is 0.09mm or ±0.045mm. If this lens is used only for daylight operations (no infrared), the maximum focal plane shift is reduced to 0.08mm or ±0.04mm which is less than 1/2000 of an inch. Considering the accuracies to which photographic cameras (from Leica) are built, this tolerance falls within those specifications.
Most camera manufacturers apply less stringent tolerances. The industry average standard is 1/1000 of an inch, with a few manufacturers like Canon, Nikon and Contax going to 1/1250 or 1/1500 of an inch. Leica cameras on the other hand continue to be made to tolerances of 1/2500 of an inch or more precisely 1/100 of a millimeter. Tolerances applied for the manufacture of lenses often needs to be much smaller. Starting with the optical glass, Leica applies a standard of ±0.0002% for the accuracy of the refractive index. This compares to the international standard of ±0.001%. The accuracy of the Abbe number, the measure for dispersion is ±0.2% for Leica compared to ±0.8% internationally. For the manufacture of individual lens elements Leica allows production tolerance of no more than ¼ lambda or ¼ of the average wavelength of light which corresponds to approximately 500 nanometers or 0.0005mm for the accuracy of the lens surface. In comparison, the tolerances applied by other lens manufacturers are ½ lambda or 0.001mm. For the production of aspherical lens elements Leica applies even tighter tolerances which cannot exceed 0.03 micrometer or 0.00003mm.
For comparison purposes the same procedure was repeated with a representative sample of a high quality photographic achromatic objective (only corrected for the primary spectrum). The Leica Elmarit-R 180mm f/2.8 lens was stopped down to f/3.4 for the test. It is strikingly evident that the different colors focus at quite different locations on the lens axis. The maximum focal plane shift from blue to infrared is 0.5mm compared to 0.09mm for the 180mm f/3.4 lens. This amounts to an increase by a factor of 5.6. The maximum focal plane shift under daylight conditions is 0.2mm as compared to 0.08mm for the 180mm f/3.4 lens. This amounts to a factor of 2.5 over the 180mm f/3.4 lens.
The most obvious advantage of the 180mm and the 450mm lenses occur when high resolution films are used. The 180mm lens would outperform any other lens with any type of photographic emulsion (or digital sensor)
The performance of the 180mm Apo Telyt-R is well known and Mr. Schantz’s article only confirms the fact that the 180mm f.3.4 Apo Telyt-R is one of the best lenses ever made.
Mr. Schantz writes the following about films:
When the improved lenses were tested with regular fine grain films, little improvement could be noticed over the performance of regular good lenses.
Through the cooperation of film manufacturers a number of film samples were tested for performance with the new lenses. Two classes of film emulsions gave promising results. These classes were (a) high definition aerial films and (b) high resolution document copy films.
These aerial films are fine grained, slow, high contrast and have extended red sensitivity. However, when processed as normally recommended by the manufacturer for aerial use, they have too high contrast for general ground photography. The same holds true for the high contrast copy films since they were designed for high contrast microfilming.
Kodak High Contrast Copy Film when processed in the POTA developer of Marilyn Levy (Levy, M., “Wide Latitude Photography,” Science and Eng. Vol. II Number I, January, February 1967) yield excellent high resolution negatives with adequate film speed. The Agfa High Contrast Copy film gives a practical combination of good resolution and emulsion speed.
According to the research done by Mr. Schantz, the best resolution obtainable with conventional film is 250 – 300 l/mm compared to 550 l/mm with the Agfa High Contrast Copy Film and 600 l/mm with Kodak 5069 and 3414 film.
Those are very impressive figures. It was soon determined that in order to make enlargements that would show this amount of detail, a special enlarger was necessary. For that reason Leitz made a modified version of their Focomat II enlarger where the standard light source was replaced with a point light source. Only such illumination systems are capable to reproducing extremely small detail from a negative.
The 35mm high resolution camera system was classified material for several years.
For other articles on this blog please click on Blog Archive in the column to the right
To comment or to read comments please scroll past the ads below.
All ads present items of interest to Leica owners.
_______________________________________________________________________
To comment or to read comments please scroll past the ads below.
All ads present items of interest to Leica owners.
_______________________________________________________________________
Buy vintage Leica cameras from America's premier Leica specialist
Buy vintage Leica cameras from
America's premier Leica specialist
Click on image to enlar
Order: info@gmpphoto.comPlease make payment via PayPal to GMP Photography
Click on image to enlarge
Order: info@gmpphoto.com
Order: info@gmpphoto.com
Click on image to enlar
Order: info@gmpphoto.com
Please make payment via PayPal to GMP Photography
Click on image to enlarge
Order: info@gmpphoto.com
Please make payment via PayPal to GMP Photography
Click on image to enlarge
Order: info@gmpphoto.com
Please make payment via PayPal to GMP Photography
No comments:
Post a Comment