Some decades ago the so called mirror or catadioptric lenses were a hit. Many lens manufacturers produced mirror lenses in focal lengths from 250 mm. up to 2000 mm. and more. All these lenses are based on an old invention.
The French priest and teacher at the famous Pocquet College in Chartres (France), Laurent Cassegrain (1629-1693) draw and constructed the first hollow/concave primary mirror, catching light beams which reflected onto a secondary mirror. This construction didn´t produce nice images but made it possible to construct a relatively short telescope with a - in those years - large aperture. Cassegrain´s design has become the base for nearly all telescopes in the world. Even today Cassegrain telescopes are used in astronomy.
Cassegrain´s design
The German astronomer Bernard Schmidt constructed in 1930 an improved mirror lens with a positive or converging lens (of light beams) in front of the primary mirror. This construction makes it possible to build a telescope with a viewing angle of approx. 15 degrees and almost without spherical aberrations.
Schmidt´s design
Dr. Dimitrii Dmitrievich Maksutov of the St. Petersburg Optical Institute (Russia) published in the Journal of the Optical Society of America (Vol. 34, No. 5 - May 1944) a design of a Cassegrain telescope improved by him. He placed in front of the primary mirror a correction lens (meniscus) neutralizing many aberrations. The Maksutov-Cassegrain-telescope was born.
Maksutov´s design
A few years earlier,
however, a Dutch scientist and general manager (1941 - 1968) of the ´Olddelft
- De Oude Delft´ Optical Industry in Delft, Holland, placed in the light beam
two kitten meniscus lenses of different kinds of glass, but with an identical
breaking index to neutralize colour aberrations. He also invented a lens system
for wide angle projection, used in Super Technorama-7 film projection. This
wide screen projection was first applied in 1959 for the presentation of the
Walt Disney cartoon ´Sleeping Beauty´.
The Nikon mirror lenses are in their construction identical. The cylindrical
lens body has in front a large lens with in its center a small dark plate. In
the back of the lens body one can see the primary mirror. This mirror catches/collects
the light beams and reflects them to the secondary mirror behind that small
plate in front. This secondary mirror enlarges the image because of its curvature
and passes the light beam through the central opening in the primary mirror
via a group of enlarging lenses to the film plane. These rear lenses are defining
the focal length of the entire mirror lens. This whole construction makes it
possible to produce a relatively short lens with a long focal length. To produce
a fast mirror lens a large front lens and primary mirror is necessary. That´s
why the Nikkor 6.3/100 cm. and the Nikkor 11/2000 mm. are huge ´barrels´.
Many mirror lens producers may say that it is an easy handling lens which enables you to make handheld pictures. Well, forget it!! A mirror lens with a focal length of 250 mm. might work, but lenses of longer focal lengths are very difficult to hold.
Because of the small aperture (5 - 11) and heavy weight (up to 10 kilo) a tripod is indispensable. Than there is the relatively long minimum distance (up to 30 meter!) and the little depth of field. A nice or disturbing effect of mirror lenses are the rings in the unsharp parts of the images. Some mirror lenses also have a so called hot spot in the center of the image, caused by a small secondary mirror. An advantage of a mirror lens is the absence of chromatic aberrations. Spherical aberrations may be eight times less than in a lens construction with an equal aperture and there is no light absorption. Finally, when using infrared films focus adjustments are not necessary.
Buying a second hand
mirror lens need attention. Inspect all mirrors thoroughly (no fungus, moisture,
etc.) and the front and rear lens. A mirror lens with a ´squint´ is difficult
to adjust. Never open a mirror lens as adjusting a mirror special tools are
needed.
Let´s have a look at the Nikon mirror lenses. A matrix of all Nikon mirror lenses
can be found here.
All Nikon mirror lenses, presented below, are based on the Maksutov-Cassegrain principle. They may be used with Nikon tele-converters (TC-200, TC-201, TC-14 and TC-14A), but it is advisable not to do it. Focusing is hardly possible and image quality is bad.
Reflex-Nikkor 5/50 cm.
Reflex-Nikkor 5/50 cm.
Presented in August 1961 this lens was rather fast. At the front 122 mm. filters
can be used, but at the rear end smaller (thus cheaper) 39 mm. filters can be
fitted. It has a scalloped focusing barrel in between the lens barrel and lens
mount. Lens specifications are engraved in the focusing ring. This black lens,
weighting in at 1.7 kilo, has been produced 9 years with an estimated production
of 12,000 lenses. Optical performance is not bad at all, given its age.
Reflex-Nikkor 8/500mm.
This lens was presented
at the 1968 Photokina Camera Fair in Cologne (Germany). It has no separate focusing
ring but the whole barrel should be turned. At the rear filters (39 mm.) can
be fitted. In fact the filter is a part of the optical system. If a tele-converter
is used that filter should be removed. In 1974 an improved (= better coated)
lens was introduced, bearing the name Reflex-Nikkor-C. Both versions have a
minimum focusing distance of 4 metre and weighting in at about 1 kilo. A redesigned
version of this lens was introduced in 1984. Minimum focusing distance is 1.5
meter and its weight is 840 gram. Important is the much better performance:
no hot spot, very nice equal light distribution and no internal reflection due
to improved coating. Of all versions approx. 60,000 lenses were produced.
Reflex-Nikkor 6.3/1000mm.
Rare 100 cm Reflex-Nikkor with reflex housing for rangefinder camera
Almost 10 kilo for a lens is quite a load. This heavy barrel was shown to the press in June 1959. A few months later is was taken into production. In front a 224mm. (!) filter can be placed; better is to use 52 mm. filters in the turret near the mount. On top of the barrel are two handles/grips, at the bottom a massive tripod plate is mounted, enabling to turn the lens barrel 90 degrees. Focusing, via a bellow (like the Zeiss Mirotar), runs from infinity to a ´minimum´ of 30 meters! This rare lens was available with a lens mount to fit Nikon rangefinder cameras (S-mount), to fit the Nikon SLR´s (F-mount) and to fit Bronica cameras. Total production is estimated at a mere 100 lenses.
Reflex-Mirror 11/1000mm.
A prototype of this lens was presented in October 1965. The front lens has a
filter thread of 108 mm., but at the back 4 filters (L39, Y48, O56, R60) are
built-in (like a revolver magazine). This lens weights in at 2 kilo. In 1974
a second version, without a filter revolver, was introduced, followed by a redesigned
version two years later. The barrel now has a tripod ring and a stylus/pin can
be fitted in the barrel making focusing a bit easier. Total production less
than 35,000 lenses.
Reflex-Mirror 11/2000mm.
From left: Reflex-Nikkor 8/500mm. - 11/1000mm. - 11/2000mm.
This rather fast but huge ´beer barrel´ of 17.5 kilo was shown at the 1970 Photokina Camera Fair in Germany. The minimum focusing distance is 20 meters and at the back 62 mm. filters can be fitted. In 1975 an improved version was introduced. It has a better coating and the hot spot disappeared. This colossus has to be mounted on a U-shaped tripod holder, which weights in at 7.5 kilo, bringing the total weight at a heavy 25 kilo!
Who maintains that
handheld photography is possible with a mirror lens?!
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