The Cooke triplet
1893 (patented 1893–94)
Dennis Taylor showed that three thin lenses, two convex and one concave, could correct the main faults of a lens. The simplest anastigmat, and the basis of many later designs.
How it began
In the 1890s the best lenses were complex and expensive. H. Dennis Taylor, an optician at T. Cooke & Sons in York, worked out in 1893 that a pair of convex lenses with a concave lens between them could do the same job with only three pieces of glass.
How it works
The concave middle element cancels the curvature of field and astigmatism that the outer convex elements produce, while their spacing balances the other aberrations. The result is an anastigmat: a lens sharp across a flat picture, not just at the centre.
Why it mattered
It was cheap to make and performed well, and it became a standard for amateur and box cameras. Its arrangement became a starting point for designers, who split and cemented its elements into new lenses, including the tessar.
Limits and what replaced it
Three elements leave little room to correct for wide angles or wide apertures. Four-, six- and many-element designs took over for demanding work.
Today
Triplets are still used in cheap cameras and projectors, and dozens of famous lenses grew out of the design.