OK, I've done a few quick test shots with both lenses: the Pentax DA 16-45 f/4 and the
Sigma 17-70 f/2.8-4.5 DC Macro. Note that, on 4-1-2008, the Pentax is available from Amazon for $330; the Sigma is available from Amazon for $370. MSRP for both lenses is much higher.
You can see the photos here:
20080401 Pentax 16-45 vs Sigma 17-70
NOTE! I am not an engineer, nor a pixel peeper. I do not claim that these comparison photos are scientifically legitimate tests. It's quite possible I've made gross errors here that badly skewed the results, so I don't want anybody to be unduly influenced by my handful of photos.
On the other hand, I did use moderate care, trying to get the same shots in pretty much the same light, with pretty much the same camera and lens settings. However, everything was done hand held (no tripod). To compensate for operator error as much as possible, I took each shot a couple of times and then selected the best one for the comparison. And I did do a little post-processing in Adobe Lightroom: slight exposure fixes, clarity, and a bit of sharpening. Some of the photos were given a black and white treatment. I justify post-processing because I post-process all of my photos and I'm interested in what I can get from the image files, not what the image files look like straight from the camera.I took my shots mainly around 35mm, and avoided the max aperture settings, too. My goal has not been to identify each lens's weaknesses but rather to try to figure out the BEST that each lens can do, given my uninspiring choice of subjects, lack of time, mediocre light, etc.
Bottom line? I am not a lens connoisseur, but to my eyes, the results from the two lenses are pretty comparable, which is to say that the Sigma 17-70 seems to be a pretty decent lens. I've stared at these photos now for an hour, and I think that it's likely the Pentax shots are "better" but the superiority is slight. And of course, the Sigma's ability to cover the focal range from 45-70mm is a big mark in its favor, indeed, it's the reason I bought the lens.
I've added a couple shots taken other lenses - one by the Pentax FA 35 f/2, one by the Sigma 18-50 f/2.8 Macro - to add two more points of comparison. I wish I still owned the kit lens so I could have thrown it in there.
I also added, at the end of the album, a couple of close-focus (I won't call 'em "macro") comparisons. Although the Pentax isn't marketed as a "macro" lens, it does fairly well at close range.
For what it's worth (which ain't much)....
Will