There have been many reviews of this lens over the net. I found the ones of Kurt Munger and Juza Ea to be amongst the most interesting.
When reading various reviews about this lens, one might wonder if everybody tested the same lens, considering how different the results can be. There seems to be a lot of sample variation amongst copies, but the lens is also particularly difficult to test. Shooting resolution targets, for example, requires extreme care: because of the angle of view, you need to be so close that any difference in flatness of the test picture will cause problems. I am literally talking fractions of millimeters here, and insuring that a test picture (typically about 1 meter base) is flat with a precision better than one millimeter on its complete surface is not easy. You can't do that by just hanging a newspaper on a wall…
The lens may also respond differently depending on the camera, for example for corner shading (vignetting). Just as Kurt Munger, I found the full aperture to be almost unusable because the corners are too dark. We both tested on a Sony A900. Juza Ea, on the other hand, says that full aperture is perfectly usable and publishes pictures with far less corner shading. He used a Canon EOS 1D MIII. It is quite possible that the two cameras respond quite differently, at 12mm the peripheral rays will reach the sensor at an angle which causes problems with digital sensors. The sensor technology is different, so could be the response.
I did not test the lens at full aperture or f/5.6, which I find barely usable. Below are the 12mm pictures at f/8 and f/11.
Center: f/8 and f/11
Corner f/8 and f/11:
(remember to click the pictures to see a 100% crop in a new window)
Conclusion? In my opinion, 12mm is quite usable at these apertures. From the other tests, we have seen that the lens is also close to the CZ 16-35 at 16mm, but that there are much better lenses at 20mm or 24mm (Sigma FTM confirms poor corner sharpness at 24mm), so it is probably best to mainly use the lens between 12mm and 16mm. Luckily, this is the most interesting part of the lens. And to put things into perspective: when Nikon made a 13mm f/5.6 about 40 years ago, it was such a difficult lens to make that they only made a few hundreds at a the price of a medium-sized car. You can read about it at the Kenn Rockwell site.
