This is a nice lens for 35mm cameras, with a built-in hood. The version I tested was M42 (Pentax screw) mount.
- Vignetting is only an issue wide open, improves a lot at f/5.6 and is basically gone by f/8
- The center of the frame is already sharp at f/3.5, and only improves a little bit at f/5
- The corners are pretty good at f/5.6 and sharpest at f/8
Please read the full review with sample images here.