[Parry]

I have the Tamron 17-50 f2.8 as my walk around lens. Very sharp but can show terrible PF which I find hard to fix in LR3.

Mine is used on demolition/construction sites so I go through one every 6 or so months due to dust and general abuse. At £279 it's what I call 'disposable'.

[grahamcoad]

Sigma 17-50 f2.8 is good. A "keeper". I haven't tried the others, but some of it's results are sharper with brighter colour and more "oomph" than both my old Pentax A 50mm f1.7 and my FA 35mm f2 lenses (both since sold) and they were much praised "Prime Lenses".

I know sharpness isn't everything, but it impressed me!

[Billgscott]

Plan ahead! The 24 - 70 is full frame, the 17 - 50 isn't. See if you can get a used 17 - 35 Sima which is also full frame.

[Medium FormatPro]

I own both the Sigma 17-50 and also the 24-70...

If one doesn't mind the (much) additonal weight, size and all of the Sigma 24-70 f2.8; and aso the 82mm front filter... The optics of the 24-70 are some of the best of any existing PK zoom that one will find out there. Sure it was made for full frame, and perhaps that day will eventually see light for Pentax, yet the lens also works perfectly with any existing Pentax digital.

How well exactly?? It's also one of the fastest focusing PK lens ever made.. The build quality is literally unmatched for anything with the exception of a Zeiss. The 24-70 is of course hsm, which is considerably better than anything in Pentax sdm.

If I had just one single choice of any lens for any existing Pentax digital camera the Sigma 24-70 would be it