Originally posted by MASTERFIRE No shopping spree now...I'll keep my Sigma 17-35 Aspherical for now and wonder forever why PENTAX does not offer a lens for K 1 users who want the versatility of a landscape friendly fl of 17mm (give or take) with zoom capability to make images with a more standard fl of 50 mm (give or take) when timing is often everything where people, wildlife, and domestic animals are around. I purchased a K1 not to crop, but to take the best images my abilities and equipment can accomplish.
You look like someone who was using a crop sensor camera body BEFORE the K1. The K1 is a Full Frame camera, thus they do have UWA DFA 15-30/2.8 and the normal DFA 24-70/2.8 lenses to cover Ultra wide to short tele.
The DFA 15-30 is 2mm wider than the Sigma 17-35. The DA*16-50/2.8 is a crop sensor lens and is NOT usable on the K1. In 35mm FF terms the lens FL translates to 24 - 75mm thereabouts on the K3, K5, K7, K30, K50, K70, KP, etc. . .
For the Sigma 17-35 on a K70 the FL is 25.5mm to 52.5mm. When you put the Sigma on to the K1, 17mm is 17mm and 35mm is 35mm.
For myself, I have the Samyang 14/2.8 for ultra wide and the DFA24-70 for normal use.
I believe that the other have been trying to explain to you the difference already.
Cheers!