The Cooke triplet is a photographic lens designed and patented (patent number GB 22,607) in 1893 by Dennis Taylor 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 lenses.
A Cooke triplet comprises a negative flint glass 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). 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 optic axis. 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 It was superseded by later designs in high-end cameras, but is still widely used in inexpensive cameras, including variations using aspheric elements, particularly in cell-phone cameras.
Read more about Cooke Triplet: Early Designs, Application
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