Large F2.8 maximum aperture. This improves the lens’s operability in photographing such subjects as landscapes and star-lit scenes, which demand high-precision focusing during extended exposures. This lens features high-grade, multi-layer HD Coating , which reduces the average reflectance in the visible ray spectrum to less than 50% of conventional multi-layer coatings.
Not used it a huge amount so far but what I have I've been pleased with the results. It is a big lens and therefore heavy, however it does produce some nice sharp images. I'm using it with a k-70 and they work well together.
It's dual purpose in that it is full frame and APSC depending which camera I use it on, there is very little curvature in the verticals with an angle of view at around 125degs with the photo sharp from corner to corner, all at a very good price