The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Pentax 16-85mm F3.5-5.6 ED DC WR and the Sigma 17-70mm F2.8-4 DC OS HSM C for their suitability to various photography scenarios:
Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Pentax 16-85mm F3.5-5.6 ED DC WR is the longer of the two lenses at 94mm. The Sigma 17-70mm F2.8-4 DC OS HSM C with a length of 82mm is 12mm shorter. On the other hand, the Sigma 17-70mm F2.8-4 DC OS HSM C has a larger diameter of 79mm compared to Pentax 16-85mm F3.5-5.6 ED DC WR's 78mm diameter.
Weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Sigma 17-70mm F2.8-4 DC OS HSM C weighs 465g, 4% (23g) lighter than the Pentax 16-85mm F3.5-5.6 ED DC WR's weight of 488g.
Below you can find the dimension and weight table of the 'Standard Zoom Lenses in Pentax KAF3 Mount for size comparison purposes.
Standard Zoom Lenses in Pentax KAF3 Mount for Size Comparison
Lens Mounts
Both Pentax 16-85mm F3.5-5.6 ED DC WR and Sigma 17-70mm F2.8-4 DC OS HSM C has the same Pentax KAF3 lens mount. Some of the latest released cameras that are compatible with the Pentax KAF3 lenses are Pentax KF, Pentax K-3 III and Pentax KP.
Sigma 17-70mm F2.8-4 DC OS HSM C is also available in Canon EF , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
Pentax 16-85mm F3.5-5.6 ED DC WR has a focal range of 16-85mm and 5.3X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 24-127.5mm when used on a APS-C / DX format camera. Sigma 17-70mm F2.8-4 DC OS HSM C has a focal range of 17-70mm and 4.1X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 25.5-105mm when used on a APS-C / DX format camera.