Plasmat lens

The Plasmat lens was developed by Paul Rudolph (1858–1935), one of the most influential optical designers of the late nineteenth and early twentieth centuries. While employed at Carl Zeiss in Jena, Rudolph created several landmark designs, among them the Protar, the Planar, and the commercially dominant Tessar lens. Royalties from these patents allowed him to retire in 1911, but post-World War I hyperinflation in Germany wiped out his savings and compelled him to return to work.^[2] Around 1920 he joined the Hugo Meyer optical company in Görlitz, where he completed a design he had begun around 1918 and which would be registered under the name Plasmat.^[3]

The new lens was a direct refinement of the earlier Dagor by Goerz, from which it inherited the double-meniscus anastigmat principle. By uncementing the rear meniscus element and allowing it to be positioned independently of the remaining cemented pair, Rudolph achieved a higher degree of correction of spherical aberration than the Dagor while retaining comparable flat-field performance.^[4] The original commercial version was introduced as a convertible (Satz-Plasmat) lens with a maximum aperture of f/4.5, targeted at view-camera users who required flexibility in focal length without carrying multiple complete lenses.^[5]

A significant practical drawback of the early Plasmat was a tendency toward flare, owing to the relatively large number of air-to-glass surfaces inherent in its separated-element construction. This limitation was substantially overcome following the introduction of anti-reflection coatings after World War II, after which the design rapidly became the standard form for moderate-wide-angle normal lenses on large-format view cameras.^[6] Meyer itself did not continue lens production after the war — the company was nationalised and eventually absorbed into VEB Pentacon — but the Plasmat configuration was taken up by other optical manufacturers and propagated in that form.^[7]

Meyer produced several distinct variants of the Plasmat alongside the original symmetrical convertible form.

The Satz-Plasmat ("convertible Plasmat") was the original commercial version. Either half-cell could be used in isolation as a single-element lens of longer focal length, albeit with some residual aberrations that become more noticeable at wider apertures or when using more sensitive panchromatic films.^[8]

The Kino-Plasmat, developed in the early 1920s, was a high-speed adaptation of the basic form for motion-picture use. The f/2 version was followed by a f/1.5 variant that was, at the time of its introduction, regarded as the fastest photographic objective in existence.^[9]

The Makro-Plasmat, introduced around 1926, was optimised for use with the then-emerging 35 mm format. Despite its name, it was not specifically intended for close-up photography; the prefix Makro here referred to adaptation for a larger image circle relative to the cine formats then in common use.^[10]

The Kleinbild-Plasmat (1931) continued Rudolph's development of the Plasmat principle for small-format cameras. In 1933, a company named Plasmat GmbH in Berlin launched the Roland, a small-format rangefinder camera fitted with a Plasmat f/2.7/70 mm lens.^[11]

Following the adoption of anti-reflection coatings, the six-element Plasmat form became dominant for large-format normal and moderate-wide-angle lenses, and remains so for the small number of manufacturers that continue to produce such optics.

The Schneider Symmar series — including the later Symmar-S, APO-Symmar, and APO-Symmar-L — is a six-element, four-group Plasmat design. The APO-Symmar generation typically covers an image circle at 72 degrees at f/5.6, sufficient for considerable camera movements on 4x5 and 5x7 formats.^[12]

The Rodenstock Sironar series — including the Sironar-N and the APO-Sironar-S — follows the same basic configuration. The APO-Sironar-S, which incorporates ED glass elements, established a coverage benchmark of 75 degrees at f/5.6 for standard Plasmat designs and offered better edge performance and colour correction compared with equivalent focal lengths from competing manufacturers.^[13]

The Fujinon W and CM-W series are likewise Plasmat designs and are regarded as optically equivalent to the Schneider and Rodenstock offerings at comparable apertures.^[14]

All of these lenses are generally offered at a maximum aperture of f/5.6, which is characteristic of the class. This aperture provides sufficient brightness for precise focusing on a ground glass while maintaining the image circle necessary for camera movements. Macro and process variants — including the Rodenstock APO-Macro-Sironar and the Schneider Macro-Symmar HM — adapt the same six-element principle for close-range reproduction work.^[15]

The Plasmat cell structure lends itself naturally to convertible use. When the front and rear cells are separated and each used individually, they function as longer-focal-length lenses, though with some residual lateral chromatic aberration and astigmatism that are most apparent at wider apertures. Contemporary convertible Plasmat sets, such as those produced by Wisner, are manufactured with modern glass types and multi-coating, and offer performance substantially superior to older uncoated convertible designs.^[16]

In the twenty-first century, Meyer Optik Görlitz revived the Plasmat name with the APO-Makro-Plasmat 105 mm f/2.7, a six-element, five-group apochromatic design drawing on Rudolph's original documentation and constructed with modern Schott optical glass and a fifteen-blade diaphragm for smooth bokeh rendering.^[17]