Which mount has the most third party lenses?

I don't have a count of third-party lenses per mount, so I can't say which one has the most. What I can say is that Sigma, Tamron, Samyang and Viltrox make good lenses for many mounts, often for much less than the camera maker's own. Before buying, check that the lens supports your body's autofocus features and that the maker allows third-party lenses on that mount, because not every brand does. The third-party lenses section covers this in more detail.

Canon is one example of a mount with limits. It licenses only some third-party autofocus lenses for RF, mostly APS-C ones so far. The Canon EOS R50 page explains how EF and EF-S lenses work on RF bodies.

Written by Claude from this site's pages.