|Cooke Triplet Lens 1916|
scanned by Geoff Harrisson (Image rights)
A triplet lens is in general any lens with three glass elements. However, it usually refers to the particular arrangement of a positive front element, a biconcave second one, and positive rear element. Such a lens was first patented as the Cooke triplet in the late 19th century. Similar lenses have been designed by many makers, and they were provided as the fixed lens on many cameras until at least 1950. It is an anastigmat, and many examples of the design are named as such. The lens is attractive because it comfortably covers the angle of view required for a standard lens, offers good correction of aberrations (or can do so: not all designs are equally good) and is cheap to make, not requiring elements to be cemented. Further, it allows front-element focusing; that is, moving the front element forwards shortens the focus distance, almost as if the whole lens had been moved. Thus on cheaper cameras, the front element is often mounted on a screw thread for this purpose, with a focus scale marked around it. This saves the provision of a more complicated unit-focusing mechanism. The Tessar, often described as a modified triplet, also allows front-element focusing.
- Other triplets include the Aldis Uno Anastigmat.