Telephoto Lens

To photograph a distant subject large, you need a long focal length - and a long lens means a long camera. In 1891 Thomas Rudolphus Dallmeyer in London, and Adolf Miethe in Germany, independently put a negative lens behind a positive one. The pair acts like a single lens of much longer focal length, yet it is physically shorter than that focal length. That is what 'telephoto' means, and nearly every long camera lens since uses it. **No patent number is cited here**: Dallmeyer patented it in Britain, and no US patent for his design was verified, so none is claimed. **You build one on an optical bench**, measure its focal length from the size of a distant image, and let a ray-trace notebook redesign it.