Evaluation: 
Conclusion: Sigma 17~70 DC Zoom for Pentax
Review: I purchased this lens to replace the standard kit lens that came with the Pentax K10D. It has proven to be of exceptional quality both optically and mechanically. I have shot it under many varied situations and it continues to give me the exact results I hoped for.
I plan to purchase the Sigma 70~300 APO MAcro zoom soon to supplement the 17~70 and expect the combo to fit 99% of my lens needs.
Evaluation:
Conclusion: Good quality Versatile Lens
Review: I purchased this lens for my Pentax K10D, and it works quite well. The zoom range is just right for most applications and the semi macro performance is handy. Performance wise it is very sharp, low CA, low vignetting and low distortions. At 17mm the performance sags a little with some distortion and when wide open some vignetting. In most cases I haven't had a problem. At 17mm, however, the distortion gets pronounced at distances less than 3 feet (1 meter), so take a few steps back if possible. Aluminum construction is nice.
Overall highly recommended!
Evaluation: 
Conclusion: Defective?
Review: I used the 17-70 for a little over a week before coming to the conclusion that mine must be defective. Where other people rave about how it focuses on a dime even in low-light levels, mine could barely be persuaded to focus on the best of days in perfect light. Very few of my pictures taken with it were what you could call crisp and none "tack sharp." Plus, it's very heavy, which made it difficult for me to handhold w/o some motion blur which only added to my problems. My father owns this lens and swears by it and his pictures ARE tack sharp with no vignetting or chromatic aberrations, so that was ultimately what lead me to conclude it defective. I returned it to 47th Street Photo, who still have yet to give me my refund, despite having had the lens since the 11th of january, and even though there will be no re-stocking as they will be sending it back to the manufacturer, they are charging me an exorbitant $50 "re-stocking" fee as I decided I didn't want another one of these behemoth lenses in my camera bag.
Evaluation:
Conclusion: Nice walk around lens, excellent color
Review: I got this lens about a month ago and it has been on my Pentax K10D for most of the time. The lens produces sharp pictures with nice colour and contrast in daylight and covers a good range from wide angle to medium telephoto. The only issue is that f2.8 is only available till 21 mm. Not the best choice if you want to shoot in low light.
Evaluation:
Conclusion: Excellent zoom that is as sharp as my prime lens
Review: I have a Pentax K10D and a Pentax SMC-FA 35/f2.0AL lens. I bought the sigma 17-70 lens to meet my basic walkaround needs based on the recommendations given by various folks on the Pentax SLR forum at dpreview. I received this lens and I am thrilled with the quality of the photos even wide open. While the 2.8 wide aperture is only valid from 17-21mm, the lens is really sharp in the entire range.
I have compared the performance of this lens with my 35mm prime at equal 35mm and equal apertures and the photographs are just as sharp with more pleasing (warmer) colors. The sigma lens is also made in Japan and the construction quality looks really good. The only two downsides are 1. The lens does not balance well on the camera (even with the BG2 grip on) so one has to place the camera LCD side down. 2. The zoom is a little stiff and one has to turn in the other way (counter-clockwise) to extend the focal length (unlike my 18-70 Nikon zoom)
Vey highly recommended. 47th street also delivered in a timely fashion and with excellent packaging.
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