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Cooke triplet |
| Cooke triplet | |
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| Introduced in: | 1893 |
| Author: | Dennis Taylor |
| Construction: | 3 elements in 3 groups |
| Aperture: | f/3.5 (early) f/2.8 (rare-earth optical glass) |
The Cooke triplet is a photographic lens design designed and patented in 1893 by Dennis Taylor who was employed as chief engineer by Cooke of York. It was the first lens system that allows 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. 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 made obsolete by later designs in high-end cameras, but is still widely used in cheap cameras.
Despite the fact that the Cooke design was patented in 1893 it seems that the use of achromatic triplet designs in astronomy appeared as early as 1765. The 1911 Encyclopedia Britannica wrote:
A similar design is used in the strong focusing synchrotron, invented first by Nicholas Christofilos in 1949, but his work was not known in the U.S., where parallel development took place.
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