The Rokinon 12mm F2.0 lens was made specifically for APS-C size sensors and smaller. This allowed the manufacture to make a compact fast wide-angle lens. The lens is made up of 12 elements in 10 groups with 2 of the elements being aspherical. The field of view is 98.9° on APS-C sensors and 83° on M43 sensors. It has a 67mm filter thread.
Build Quality: The lens is well constructed and relatively small. The lens mount is metal and the outside is plastic. The plastic is high-quality and feels tougher than the Sony lenses. The aperture ring is a little small, but provides nice clicks with half stop adjustments. The aperture and focus rings are well-dampened, needing both fingers to turn. Close focus is a little less than .66m and it focuses a little past infinity, but there is a mark for infinity. The lens hood is a little cheap and also has a little play when mounted. The build quality is excellent, except for the lens hood.
Performance: The lens performed very well for my uses. I’m only going to shoot landscapes and nightscapes with this lens. The center is sharp at F/2.0, peaking at about F/5.6. The corners are a bit soft wide open, but sharpen up dramatically at F/4.0. When focused to infinity at the stars, the corners were surprisingly good wide open. The coma was well controlled too. It can do up to a 25 second exposure without getting star trails. There is little distortion, and it shouldn’t be a problem fixing it in post. It has strong vignetting at F/2.0, but improves at F/2.8. Overall, it has excellent performance for a $400 lens.