The Pentax *ist D and DS have the pentax K mount, which means they are backward compatible with all K mount lenses made by Pentax (and many other manufacturers). With an adaptor, you can also use the even older screwmount lenses! The A series primes are particularly sought after, and in this Canon/Nikon centric world, they are getting hard to find. I was lucky enough to find a 50mm f2.0 for the grand sum of $49 in a hock shop... here's some shots on the day I got the camera. Levels adjusted, and some USM applied.