Cooke triplet

At the time, the Cooke triplet was a major advancement in lens design. For one wavelength, the design can correct, using only three elements, spherical aberration, coma, astigmatism, field curvature, and distortion.[2] However, Kingslake noted "there is no symmetry to help the designer, and there is no control over distortion", leading to a trial-and-error design process.[4]^{: 104–105} It was surpassed by later designs in high-end cameras, including the double Gauss design, but is still widely used in inexpensive cameras, including variations using aspheric elements, particularly in cell-phone cameras.

The Cooke triplet consists of three separated lenses positioned at the finite distance. It is often considered that the triplet is one of the most important discoveries in the field of photographic objectives

— The Cooke triplet optimizations (2002)[2]

T. Cooke & Sons were reluctant to manufacture the lens, possibly because of the difficulty in grinding the strong central negative element, and the design was licensed to Taylor, Taylor and Hobson, who named the product the Cooke triplet.[4]^: 105

Starting from the early 1900s, the Cooke triplet was adopted by other major optical lens manufacturers and were produced for many decades.

• Cooke triplet and derivative design diagrams
• 
  Cooke triplet (Taylor, 1893)[9]
• 
  Voigtländer Heliar (Harting, 1900)[10]
• 
  Voigtländer Dynar (Harting, 1903)[11]
• 
  Taylor Hobson Speedic (Lee, 1924)[12]
• 
  Zeiss Sonnar (Bertele, 1924)[13]

Arthur Cox noted that anastigmat lenses were "almost exclusively, the logical development of two main types, the symmetrical lens, and the Cooke triplet of H. D. Taylor."[14]^: 241 One of the first derivatives was the Voigtländer Heliar, developed by Hans Harting in 1900 as a symmetrical modification of the original Cooke triplet.[4]^: 106 Harting continued to develop the lens, resulting in the Dynar (1903), whose design was later adopted as a new version of the Heliar after World War I.[4]^{: 107–108} Anecdotal evidence indicates that a Heliar was used to take official portraits of Emperor Hirohito.[15] Although the contemporary Zeiss Tessar (1902) is said to be a development of the Cooke triplet, it is more accurately considered a parallel development descended from earlier, four-element designs by Paul Rudolph, including the Protar and Unar.[16]^: 90

To increase the lens speed, either the rear element was further split into two, as patented by Edward Bausch in 1900 and H.W. Lee as the Speedic in 1924, or a positive meniscus element was inserted into the front air space, as in Ludwig Bertele's enduring Ernostar and Sonnar designs for Ernemann and Carl Zeiss AG, respectively.[4]^{: 108–111}

Several of the early lenses used with the Leica camera were derived from the Cooke triplet by splitting one or more of the three elements into a cemented doublet, including the Elmar, Elmarit, Hektor, and Thambar.[4]^: 113