Secondary lens
Encyclopedia
In photography
, a secondary lens is a lens
designed to be used in conjunction with another lens, called the primary lens.
A secondary lens may be designed to be used either in front of the primary lens, between it and the subject, or behind the primary lens, between it and the film.
Secondary lenses are typically simpler in mechanical design than other photographic lenses. Normally but not always there is no provision for control of focus
or aperture
, as these are achieved by the controls on the primary lens.
All of these typically mount on the filter ring of the primary lens.
Front-mounting secondary lenses are commonly used to provide wide-angle and telephoto lenses for cameras which lack the facility to change the primary lens, such as consumer video camera
s and camcorder
s, and some older consumer medium format cameras.
. A typical teleconverter has a male and a female lens mount of the same type, to allow it to be used between a primary lens and a camera body of the matching system. The resulting combination has a greater focal length
than the primary lens alone, and the same absolute aperture
, so its f-number
is increased accordingly.
Some teleconverters are dedicated or matched to particular primary lenses, normally one or more lenses by the same manufacturer as the teleconvertor, and when used in this combination deliver quality comparable to the primary lens. Others are general-purpose, and in use typically offer less image quality then a primary lens of the same quality, but allow a camera kit to offer a wider variety of focal lengths with a smaller number of lenses, saving in both cost and weight.
It is not normally possible to construct a similar wide-angle adaptor, as the angle of view
of the primary lens is matched to the film format
for which is designed, and unwanted light is typically stopped by baffles. Any device similar to a teleconvertor designed to use the primary lens for wider angles than those for which it is designed is therefore likely to cause vignetting
.
Specialised secondary lenses may be used behind the primary lens to adapt a lens of one camera system and/or film format to another. These are called active adaptors or converters. For example, a Pentax screw mount lens cannot normally be mounted on a Nikon F-mount
camera, as the Nikon bayonet mount is further from the film plane
than the Pentax screw mount, leaving no room for a mount adaptor. However active adaptors are available containing a (not very powerful) teleconverter, which makes the combination mechanically possible, albeit with a small amount of cropping at the image edges and loss of connection of the camera to any lens automation.
The optical components of these adaptors may reduce or increase the focal length of the combination, or they may affect only the back focal distance of the combination, leaving the effective focal length for purposes of photographic composition unchanged.
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, a secondary lens is a lens
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...
designed to be used in conjunction with another lens, called the primary lens.
A secondary lens may be designed to be used either in front of the primary lens, between it and the subject, or behind the primary lens, between it and the film.
Secondary lenses are typically simpler in mechanical design than other photographic lenses. Normally but not always there is no provision for control of focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
or aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
, as these are achieved by the controls on the primary lens.
Lenses used in front of the primary lens
Examples of secondary lenses designed to be used in front of the primary lens are:- FisheyeFisheye lensIn photography, a fisheye lens is a wide-angle lens that takes in a broad, panoramic and hemispherical image. Originally developed for use in meteorology to study cloud formation and called "whole-sky lenses", fisheye lenses quickly became popular in general photography for their unique, distorted...
adaptors - Corrected wide angleWide-angle lensFrom a design perspective, a wide angle lens is one that projects a substantially larger image circle than would be typical for a standard design lens of the same focal length; this enables either large tilt & shift movements with a view camera, or lenses with wide fields of view.More informally,...
adaptors - Telephoto adaptors such as teleside converterTeleside converterA teleside converter is a secondary lens which is mounted on the front of a photographic lens to increase the effective focal length of the lens they are attached to. They are used on cameras and video cameras with non–interchangeable lenses to increase the magnification of the image...
s - Close-up lensClose-up lensIn photography, a close-up filter, close-up lens or macro filter is a simple secondary lens used to enable macro photography without requiring a specialised primary lens...
es
All of these typically mount on the filter ring of the primary lens.
Front-mounting secondary lenses are commonly used to provide wide-angle and telephoto lenses for cameras which lack the facility to change the primary lens, such as consumer video camera
Video camera
A video camera is a camera used for electronic motion picture acquisition, initially developed by the television industry but now common in other applications as well. The earliest video cameras were those of John Logie Baird, based on the electromechanical Nipkow disk and used by the BBC in...
s and camcorder
Camcorder
A camcorder is an electronic device that combines a video camera and a video recorder into one unit. Equipment manufacturers do not seem to have strict guidelines for the term usage...
s, and some older consumer medium format cameras.
Teleconverters
The most common secondary lens used behind the primary lens is the teleconverterTeleconverter
A teleconverter is a secondary lens which is mounted between the camera and a photographic lens. Its job is to enlarge the central part of an image obtained by the objective lens...
. A typical teleconverter has a male and a female lens mount of the same type, to allow it to be used between a primary lens and a camera body of the matching system. The resulting combination has a greater focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
than the primary lens alone, and the same absolute aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
, so its f-number
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
is increased accordingly.
Some teleconverters are dedicated or matched to particular primary lenses, normally one or more lenses by the same manufacturer as the teleconvertor, and when used in this combination deliver quality comparable to the primary lens. Others are general-purpose, and in use typically offer less image quality then a primary lens of the same quality, but allow a camera kit to offer a wider variety of focal lengths with a smaller number of lenses, saving in both cost and weight.
It is not normally possible to construct a similar wide-angle adaptor, as the angle of view
Angle of view
In photography, angle of view describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term field of view....
of the primary lens is matched to the film format
Film format
A film format is a technical definition of a set of standard characteristics regarding image capture on photographic film, for either stills or movies. It can also apply to projected film, either slides or movies. The primary characteristic of a film format is its size and shape.In the case of...
for which is designed, and unwanted light is typically stopped by baffles. Any device similar to a teleconvertor designed to use the primary lens for wider angles than those for which it is designed is therefore likely to cause vignetting
Vignetting
In photography and optics, vignetting is a reduction of an image's brightness or saturation at the periphery compared to the image center. The word vignette, from the same root as vine, originally referred to a decorative border in a book. Later, the word came to be used for a photographic...
.
Cross-system adaptors
Specialised secondary lenses may be used behind the primary lens to adapt a lens of one camera system and/or film format to another. These are called active adaptors or converters. For example, a Pentax screw mount lens cannot normally be mounted on a Nikon F-mount
Nikon F-mount
The Nikon F-mount is a type of interchangeable lens mount developed by Nikon for its 35 mm SLR cameras. The F-mount was first introduced on the Nikon F camera in 1959, and features a three lug bayonet mount with a 44 mm throat and a flange to focal plane distance of 46.5 mm...
camera, as the Nikon bayonet mount is further from the film plane
Flange focal distance
For an interchangeable lens camera, the flange focal distance of a lens mount system is the distance from the mounting flange to the...
than the Pentax screw mount, leaving no room for a mount adaptor. However active adaptors are available containing a (not very powerful) teleconverter, which makes the combination mechanically possible, albeit with a small amount of cropping at the image edges and loss of connection of the camera to any lens automation.
The optical components of these adaptors may reduce or increase the focal length of the combination, or they may affect only the back focal distance of the combination, leaving the effective focal length for purposes of photographic composition unchanged.