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 variety 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
Photos: 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 Naval 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
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 and Nikon 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.
Of the three lenses, the
180mm f.3.4 ELCAN lens proved to be the most practical one which resulted in a
declassification, allowing Leitz to make the 180mm part of their lens line for
the Leica reflex cameras. It entered the
market in 1975 as the 180mm f/3.4 Apo Telyt R.
Even today it ranks as one of the best lenses ever made for general
photography.
___________________________________________________________________________
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