I have a 28/2.8 M lens and find it quite good. There was a 28/3.5 SMC Pentax which was supposed to be the same optically as the earlier screw mount 28/3.5. It often sells for more than the later 28/2.8 M. If I am not mistaken the 28/3.5 M was sold only in non-U.S. markets. Minolta sold a 28/3.5 MD lens in non-U.S. markets. In the U.S. you could get a 28/2 or a 28/2.8. The differece in price between the f/2.8 and the f/3.5 was not significant enough in the U.S. for both models to be offered.