I am looking at maybe getting a screwmount Super Tak 135, and am trying to decide between the f/2.5 and 3.5. I've had the 3.5. It was fine, especially stopped one click to 4.0. The extra speed on the 2.5 would be nice, and a more standard filter width (58mm) but I don't know much about the optical characteristics of that lens. Anyone ever compare these two, either on Pentax cameras or especially adapted to m43?