Another adapted lens expert on another forum pointed out that the aperture in the original camera may be the thing limiting these lenses to a fixed f/2.8 and not the lens itself. That got me thinking, and I did a few tests on mine comparing them to existing lenses. The results were surprising: The 24mm was exactly f/2.8, the 50mm was 1/3 stop faster, and get this - the 20-40mm is one whole stop faster at 25mm! The zoom lens varies +/- 1/3 stop from 40 to 20mm, and at 20mm it is about f/1.8! This could explain why there are so many reports of it not being as sharp as the others. In any case, I bought the 20-40 for $21, so I'm happy.