I have the Pentax 17-70 and 16-45; both excellent lenses IMO and not a lot to choose between them in terms of image quality, the 17-70 has a creamier bokeh and is maybe fractionally sharper. I've not used the Sigma. As I've said elsewhere I think both are eclipsed in terms of image quality - though only marginally - by my other standard zoom, the FA24-90.

I have a 16-45, 24-90 and (acquired more recently) 17-70 as 'standard walkabouts', all Pentax. Before buying the 17-70 I would either carry both lenses or decide on one or the other, depending on where I was walking about. Now the 17-70 is on the camera most of the time, sometimes with my 55-300 in the bag - though when I don't need the wider angle I actually prefer the 24-90 as it produces very slightly sharper results and there's less distortion at the long end. I haven't used the 16-45 since buying the 17-70, though if 45mm is long enough the image quality is just as good at not much more than half the price.

I have a friend who is a keen enthusiast and is very happy with her K100D which she has had from new - she likes my K20D but isn't worried about upgrading, and prefers the smaller camera for taking walking. A colleague (an architect) uses a K200D for all his architectural shots for portfolios, presentations etc. On the other hand I wouldn't want to swap the other way - if my K20 expired I would be looking for a K5, or at least another K20.

I've just come back from two weeks in Madeira and Porto Santo (city, coast, and mountainous countryside) where I deliberately took only my DA 17-70. Most of the time it did everything I wanted, and avoided me carrying extra lenses (the K20 + one lens is heavy enough on 10-15 mile hikes in my view). It's doing what I want, which is to give me reasonably good IQ and range with one lens when my main objective is walking and exploring rather than specifically photography. A couple of times I would have liked the 55-300 with me too.

I tend to agree, I bought the 17-70mm because I was often carrying both the 16-45 and the 24-90 on walks. I can get away with the latter in open country or along the coast, but 24mm seems too limiting in town or mountains.

Strangely I've only used the 24-90 on my ME Super once - there is something satisfying with a camera from that era in changing between old primes - maybe because that's how I started off!

It should be OK now, apart from my comma:

http://www.sld.demon.co.uk/lenses.pdf

Thanks John for editing the link. My web site seems to be inaccessable at the moment though.

Now I have three 'standard-range' zooms (in addition to the 18-55 that came with my K20D) I thought I'd post a brief comparison based on some test shots I did at 17mm, 35mm and 70mm x f/4, f/8 and f/16.

There is more detail here (no images unfortunately) http://www.sld.demon.co.uk/lenses.pdf, but in summary:

For image quality the 24-90 was consistently the best performer at f/4 and f/8, with the DA lenses catching up to an extent at f/16. The difference between the 16-45 and 17-70 was less marked, with the 17-70 slightly sharper in some conditions and with a better bokeh. At 70mm the 24-90 exhibited less distortion (pincushioning) than the 17-70.

So my verdict is basically that the 17-70 replaces the 16-45 for most purposes, but I will be keeping the 24-90 for occasions when I want a light walkabout lens and don't need the wide end.

Interesting to see this comparison broadly agrees, though it doesn't include the 17-70:

http://ricehigh.blogspot.com/2010/03/standard-zoom-mega-shootout-da-vs-faj.html

Stan

I have one and it's an excellent lens - it is sharper than both the DA16-45 and DA17-70 wide open at all comparable focal lengths. It doesn't feel as well-made as the 17-70 in particular, but that doesn't affect the image quality. I will try to post a comparison later in the week.

I don't have any experience of the Tamron, but I changed from a Sigma 70-300 (macro version) to a Pentax 55-300 about a year ago, and the difference in image quality was immediately noticeable even on my laptop screen. Out of the zooms listed below it seems to be the sharpest, even at 300mm.