A number of third party companies have been making lenses that work in the Nikon F (DX/FX) and Canon EF/EF-S mounts.
The big three of these companies making autofocus lenses and that are most available to consumers are: Sigma, Tamron, and Tokina, But Voigtlander and Zeiss have also been a long term provider of manual focus lenses for these mounts. More recently, we have a number of Chinese, German, Japanese, and Korean optical companies providing additional lenses now, though many of these are also manual focus.
