SDM (Supersonic Direct Drive Motor) :Shoot discreetly with in lens, auto focus motor operation for quick, smooth and quiet performance. WR (Weather Resistant) : Designed with eight specially designed seals to prevent intrusion of water; The lens also features Quick Shift.
It's a large lens but this to be expected in a large aperture zoom. It's a sharp lens and the build quality is very good. I use this lens for photographing waterfalls and have enjoyed it. It does require large filters because of the bulbous front element. That being said it's a great lens.
This thing is built like a tank and takes absolutely amazing shots. Yes, it's a bit pricey, but I think worth it for the shots it takes. Perfect match for my k-1