Sigma 17-50 f2.8 EX DC Pentax Mount Quick Review
This lens is equivalent to 25mm - 75mm focal range and about an f4 aperture in the depth of feild department on Full Frame
Calling the lens an f4 also makes sense because of the noise performance increase on full frame as well as the reduced depth of feild.
Now, I keep making refrence to full frame, and this is an APSC lens? Well here's the lowdown... If you own an APS-C camera in particular from Pentax this lens is loosely equivalent to the 'standard zoom' focal length of 24-70mm for both Canon and Nikon full frame bodies. Also the fast aperture is typical of a professional zoom in that range so you could argue that this is the poor mans 24-70 f2.8 and to be frank it sort of is. I'm not going to do all the crazy lens tests and graphs that everyone else does but I can show you some photos this lens produces and say that in its focal range pentax don't make a sharper zoom at f2.8. It also focuses quicker than their offerings too.
I have to say I simply love this lens on Pentax it really is a fantastic piece of glass and couldn't recommend it enough
Don't take my word for it though, Please have a look through the photos :-)