Sekundaarikäämit

Sekundaarikäämit, literally translated as secondary lenses, are optical elements used in conjunction with primary lenses to modify or enhance the imaging system in telescopes, microscopes, cameras, and other optical instruments. These auxiliary lenses are mounted after the main objective and before the final image plane. Their purpose is to correct for aberrations, adjust magnification, change field of view, or alter focal length without needing to redesign the primary lens.

Optical functions of sekundaarikäämit include field flattening, coma correction, and astigmatism reduction. In compound microscopes, a secondary lens can balance the numerical aperture of the system, improving depth of field and chromatic alignment. In telescopic binoculars, a secondary element lengthens the optical path, allowing for longer focal lengths within compact housings and enabling high magnification while keeping aberrations minimal.

Secondary lenses are typically made from high‑dispersion glass or low‑index materials, depending on the aberration they are designed to correct. Anti‑reflection coatings reduce ghosting and light loss. Because they are positioned further from the entrance pupil, sekundaarikäämit generally introduce fewer optical distortions than aspheric elements placed in the main objective.

Historically, the use of secondary lenses dates back to the 18th‑century refracting telescopes, where Steinheil and others added auxiliary elements for field correction. The modern creative optics industry frequently integrates secundaarikäämit in custom camera lenses to achieve unique distortion profiles or specialized field‑of‑view requirements.

Manufacturers produce standardized secondary elements such as tube adapters, field flattener rings, and apocromic correctors. These components are sold separately to allows retrofitting older systems. In professional optics, the integration of secondary lenses remains a cost‑effective strategy to upgrade performance without the expense of a complete optical redesign.