When upgrading from the ubiquitous 18-55mm kit lens, many photographers naturally look at the 17-70mm range. Lenses in this range are very versatile and typically offer a good balance of performance and price. Three lenses are currently available for Pentax cameras in this range, one DA 17-70mm F4 from Pentax and two from Sigma (one new and one old). These lenses have many similarities, but also have their share of differences, including size, focus mechanism, maximum aperture and close focus capabilities. All three lenses are generally well regarded in the Pentax community.
The first of the two Sigma lenses, the 17-70mm F2.8-4.5 DC Macro, was launched in 2006. Compared to the newer Sigma 17-70mm F2.8-4 DC Macro OS HSM from 2010, it's got a different optical formula and lacks HSM (silent) focusing. While the new lens doesn't offer quite as much macro magnification (1:2.7 instead of 1:2.3), it is half a stop faster at the long end. Optical performance is generally considered to be very similar between these two lenses.
This review will compare all three lenses to illustrate their differences, their strong points and weaknesses. Each lens will first be presented at length, and we will take a look at its physical characteristics, operation, and performance. A comparative section will then analyse each lens’s merits and flaws when compared to the other. We will look at physical differences, operation, and optical performance.