One of the nice things about Canon FD lenses is that there are a lot of third-party lenses floating around, and it's cheap to do some bottom-feeding on eBay and experiment. I got a Kiron 80-200 zoom that way---I think it cost me ten bucks---that's stupidly sharp for a zoom lens and competitive with some respectable primes. There's a lot of rebranding (Vivitar never made lenses themselves, for instance, just branded lenses sourced from a bunch of Japanese manufacturers) and a lot of uncertainty, which kind of adds to the fun---you may get a terrific lens for cheap, you may get something so bad that it's kind of entertaining, or anything in between.