2.3 Afocal Systems: General

An afocal system is one which images an infinitely distant object at infinity. It is called afocal because it has no focal length (or an infinite one; either concept can be a nuisance). Telescopes, beam expanders, power and field changers, as well as telephoto and wide-angle attachments, are examples of afocal systems.

An afocal system can form an (infinitely distant) image which subtends an angle which is larger or smaller than the angle subtended by the object. The ratio of the angle subtended by the image to that subtended by the object is the angular magnification, or magnifying power, MP, of the afocal system. If the image is enlarged, as in a telescope, MP > 1.0; if the image is smaller than the object, MP < 1.0. When the image appears erect, MP is positive; a negative MP indicates an inverted image.

The magnification MP, the diameters of the entrance pupil P and exit pupil P ?, and the angular size of the real (object side) field A and the apparent (image side) field A ? are all related by the following equation:

 (2.1)

Equation (2.2) gives this relationship for finite (as opposed to infinitesimal/paraxial) field angles:

 (2.2)

All afocal systems must obey these relationships. For example, in a typical 6 binocular, the exit pupil diameter must be one-sixth of the entrance pupil diameter, and the apparent (image) field must be six times the real (object)...

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Telecentric Lenses
Telecentric lenses are used to provide images independent of an object’s distance, or to prevent wide ranges of angles of incidence. There are three basic types of telecentric lenses: object-space, image-space, and double telecentric.
Optical Lens Assemblies
Optical lens assemblies are composed of a number of lenses mounted for a particular imaging function.  These can include telecentric lenses, collimators, beam expanders, zoom lenses, etc.
Magnifiers

Magnifiers enlarge the appearance of objects when viewed through a lens. These facilitate the inspection of fine details and the assembly of small components.

Zoom Lenses
Zoom lenses are mechanical assemblies of lens elements that provide the ability to vary focal length and angle of view.
Microscopes
Microscopes are instruments that produce magnified images of small objects

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