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. The Cooke triplet is noted for being able to correct the Seidel aberrations.

It is recognized as one of the most important objective designs in the field of photography. The lens designed, invented by Dennis Taylor 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.

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.

In this sample, we use the Multi Start tool in CAXCAD to get the design more easily. 

The merit function is combined with EFFL and SPOT targets.

Start point two made by CAXCAD automaticly

Start point two made by CAXCAD automaticly

The Derivative Increments is very good

We only take about 0.3 second to optimize the lens.

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 It was superseded by later designs in high-end cameras, but is still widely used in inexpensive cameras, including variations using aspheric elements, particularly in cell-phone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.

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