The Plasmat lens is a widely used and long-established

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

type invented by Paul Rudolph in 1918, especially common in

large-format Large format photography refers to any imaging format of or larger. Large format is larger than "medium format", the or size of Hasselblad, Mamiya, Rollei, Kowa, and Pentax cameras (using 120- and 220-roll film), and much larger th ...

photography. It provides high correction of aberrations with a moderate maximum

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

(e.g. ). It is a specific instance of the Dagor type double-meniscus anastigmat. Double-meniscus anastigmats use widely separated positive and negative surfaces, generally thick meniscus lenses, to achieve a flat field. The most basic form is two sharply curved meniscus elements located symmetrically about a stop. Further refinement of the form replaces the two simple meniscus lenses with achromats for chromatic correction. The Dagor type further refines these achromats into triplets with the following design parameters: a high-index, doubly convex (DCX) lens cemented to a medium-index, doubly concave (DCV) lens cemented to a low-index meniscus lens. Up to this point, all refinements have maintained symmetry about the stop. The Plasmat further refines the Dagor form by uncementing the meniscus, allowing for placement away from the first two elements and removing the criterion of symmetry. In its most basic form, it is symmetrical and consists of two cemented groups of three lenses each. The innermost element in each group is a positive meniscus, the outermost is biconvex, and there is a biconcave element between them. The Plasmat lens is made in many variants, e.g. departing from exact symmetry, adding a lens to one or both groups, or separating the innermost or outermost element from the rest of the group. Standard lenses for large-format cameras are generally of the Plasmat type, as are many macro lenses.

Convertible lens A camera lens, photographic lens or photographic objective is an optical lens or assembly of lenses (compound lens) used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capa ...

es for large-format photography often consist of Plasmat cells.

References

"A History of the Photographic Lens"
Rudolf Kingslake p. 95 Retrieved from books.google.co.jp February 20, 2008 *Sidney F. Ray, ''Applied Photographic Optics'', London and Boston: Focal Press, 1988, . Also later (corrected) editions. *Warren J. Smith "Modern Lens Design" Photographic lenses {{optics-stub

See also

References