I haven’t been able to test this lens much as I received it recently. So far it seems pretty good.
The critical stats of the lens are it has a focal length from 21mm to 35mm controlled by a push-pull function, pretty much the only super wide angle zoom, at least in OM mount, that isn’t 2-touch. The aperture is variable from f/3.5 (21mm) to 4.2 (35mm). The minimum aperture is f/22 (21mm) and f/27 (35mm) and is controlled in half stop increments. The lens has a maximum reproduction ratio of 12.5:1 focusing at 50cm (1.6ft). The lens is composed of 12 elements in 12 groups with a diameter of 82mm, a length of 74mm and weight of 450 grams. The lens comes with a bayonet hood that covers the top and bottom of the lens, not quite a true petal hood as there are no side protrusions. The lens has a 77mm filter size.
I did have to ‘fix’ the lens as at any focal length less then 21mm it wouldn’t quite reach infinity. On this lens and also several other Sigma lenses the focusing ring isn’t set by screws, but by tape. The tape actually appears like HVAC metalized tape and could probably be replaced by that if you wanted. It is located under the rubber grip, you just remove the tape, twist the top part of the barrel so that infinity focus is actually achieved, re-tape the barrel and replace the rubber grip. In this case the barrel had to be adjusted about 1.5 degrees so it can focus just a hair past infinity.
Distortion, distortion. Its not well controlled, especially at 21mm. At 21mm it shows pronounced wave from distortion, especially on the right side of the frame alternating from barrel distortion in the center of the frame to about neutral on the left side. The right side of the frame starts as barrel, but transitions to pincushion by about 80% of the way out to the edge of the frame. At 35mm the distortion is much better controlled with only a small amount of barrel distortion. The distortion at 21mm is evident in real world pictures, but generally tends to lend more of a ‘fisheye’ look to the picture then anything (though not nearly that pronounced, it just has a bit of it), the wave from isn’t normally visible. At 35mm distortion isn’t visible in real world pictures unless taking a picture of a brick wall, up close.