Plasmat lens

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 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.

Convertible lenses for large-format photography often consist of Plasmat cells.