afocal system

{{Short description|Optical system that produces no net convergence or divergence of the beam}}

File:Système afocal téléobjectif.svg

In optics, an afocal system (a system without focus) is an optical system that produces no net convergence or divergence of the beam, i.e., has an infinite effective focal length.{{Cite web|url=https://books.google.com/books?id=4nxMduNT8-gC&dq=afocal&pg=PA379|page=379|title=Handbook of Optical Design|first1=Daniel|last1=Malacara-Hernández|first2=Zacarías|last2=Malacara-Hernández|date=September 21, 2003|publisher=CRC Press|via=Google Books}} This type of system can be created with a pair of optical elements where the physical distance d between the elements is equal to the sum of each element's focal length f_i (d = f₁+f₂). A simple example of an afocal optical system is an optical telescope imaging a star, the light entering the system is from the star at infinity (to the left) and the image it forms is at infinity (to the right), i.e., the collimated light is collimated by the afocal system.{{Cite web|url=https://books.google.com/books?id=Jg1JW4kWD1UC&dq=afocal++without+focus&pg=PA38|page=38|title=Optical Imaging and Aberrations: Ray geometrical optics|first=Virendra N.|last=Mahajan|date=April 6, 1998|publisher=SPIE Press|via=Google Books}} Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam, increasing magnification. The magnification of such a telescope is given by

:$M\; =\; \backslash frac\{f\_2\}\{f\_1\},$

Afocal systems are used in laser optics, for instance as beam expanders, Infrared and forward looking infrared systems, camera zoom lenses and telescopic lens attachments such as teleside converters,{{Cite web|url=https://books.google.com/books?id=hcq_40I_7egC&q=afocal&pg=PA190|page=187|title=Optics in Photography|first=Rudolf|last=Kingslake|date=April 6, 1992|publisher=SPIE Press|via=Google Books}} and photography setups combining cameras and telescopes (Afocal photography).