Amongst 135mm lenses, this is definitely not the fastest at f/3.5. But it performs very nicely right from wide open, with only a slight drop in contrast. The relatively narrow aperture, and lack of focus motor and electronics allows it to be small and lightweight. A short built-in hood comes in handy when shooting in the drizzle, though on my version there's not really any friction there to hold the hood open, it just falls back into its mount if the camera is tilted upwards.
The version I shoot with is M42 (Pentax screw) mount. I mount it to the camera using a Fotodiox M42 adapter, which has an "AF-confirm" chip on board to provide EXIF data (focal length and maximum aperture), and enable the camera's "AF confirmation" light that lets you know when the image is in focus.
- 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/5.6.
- The corners are pretty good at f/5.6 and sharpest at f/8.
These images were shot on a 5D Mark III.
Chromatic aberration correction has not been applied to any of these images, but can be trivially and completely corrected in Lightroom.
Whoops, sorry, my brick wall is not nearly as flat as I thought it was when I took the shots.