I picked up a Vivitar Series 1 100-500mm f5.6-8 at a car boot sale. Cost me £15, I think it's going to replace my cheapo 500mm as my main long lens - can't believe that a zoom is giving better results than a prime lens, but the series 1 lenses weren't cheap and it's still in very good condition so I suppose it's possible. This is cropped from the full frame on a D90. 1/800 f16, 500mm.