The Cooke triplet is a

photographic lens A camera lens (also known as photographic lens or photographic objective) 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 capab ...

designed and

patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A ...

ed (patent number GB 22,607) in 1893 by

Dennis Taylor Dennis Taylor (born 19 January 1949) is a Northern Irish retired professional snooker player and current commentator. He is best known for winning the 1985 World Snooker Championship, where he defeated the defending champion Steve Davis in a ...

who was employed as chief engineer by T. Cooke & Sons of York. It was the first lens system that allowed elimination of most of the optical distortion or aberration at the outer edge of the image. The Cooke triplet is noted for being able to correct the Seidel aberrations. It is recognized as one of the most important

objective Objective may refer to: * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pronoun that is used as a grammatical object * Objective Productions, a Brit ...

designs in the field of photography. The lens designed, invented by

but named for the firm he worked for, consists of three separated lens elements. It has two biconvex lenses on the outer and a biconcave lens in the middle. The design took a new approach to solving the optical design issues, and the design was presented to the Optical Society of London.

Design

A Cooke triplet comprises a negative

flint glass Flint glass is optical glass that has relatively high refractive index and low Abbe number (high dispersion). Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractiv ...

element in the centre with a crown glass element on each side. In this design, the sum of all the curvatures times indices of refraction can be zero, so that the field of focus is flat (zero

Petzval field curvature Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis (or a non-flat object past the hyperfocal distance) cannot be brought properly into focus on a flat image plan ...

). In other words, the negative lens can be as strong as the outer two combined, when one measures in dioptres, yet the lens will converge light, because the rays strike the middle element close to the

optical axis An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light pro ...

. The curvature of field is determined by the sum of the dioptres, but the focal length is not. At the time, the Cooke triplet was a major advancement in lens design. It was superseded by later designs in high-end

camera A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with ...

s, but is still widely used in inexpensive cameras, including variations using aspheric elements, particularly in cell-phone cameras. The Cooke triplet can correct, with only three elements, for one wavelength,

spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strik ...

coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...

astigmatism Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at n ...

, field curvature, and

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signa ...

Application

The triplet soon became a standard in lens design still used with low-end cameras today. The main optical manufacturers often further developed the original Cooke triplet (e.g., the Zeiss Triotar) that were produced for many decades.

Binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

as well as refracting telescopes often use triplets. The same holds for many projection lenses, e.g., for 35 mm

slide projector A slide projector is an opto-mechanical device for showing photographic slides. 35 mm slide projectors, direct descendants of the larger-format magic lantern, first came into widespread use during the 1950s as a form of occasional hom ...

s. Image: Rollei B 35 black.jpg, ''Triotar''-Triplet on a

Rollei Rollei () was a German manufacturer of optical instruments founded in 1920 by and in Braunschweig, Lower Saxony, and maker of the Rolleiflex and Rolleicord series of cameras. Later products included specialty and nostalgic type films for the ...

scale-focus camera Image: Ed LiesegangPatrinast1-28 85 d37 zebra top.jpg, Projection objective ''Patrinast'' for a 35 mm slide projector by Ed. Liesegang; 1:2.8/85 Image: Will-Wetzlar-Maginon 1-2.8 100 d42Obl.jpg, Projection objective ''Maginon'' by Wilhelm Will, Wetzlar, 1:2.8/100 The Cooke triplet has provided the basis for additional designs, including a derivative with five lens elements.

References

{{Reflist

External links

Explanation of triplets from Fundamental Optical Design
Photographic lens designs

Design

Application

See also

References

External links