The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 18-50mm F2.8-4.5 DC OS HSM and the Pentax 17-70mm F4.0 AL SDM for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is a crucial factor to consider when comparing two lenses. Pentax 17-70mm F4.0 AL SDM is the longer of the two lenses at 94mm. The Sigma 18-50mm F2.8-4.5 DC OS HSM with a length of 89mm, is 5mm shorter. Besides being longer, the Pentax 17-70mm F4.0 AL SDM also has a larger diameter of 75mm compared to the Sigma 18-50mm F2.8-4.5 DC OS HSM's 74mm diameter.
The weight of a lens is equally significant as its external dimensions, particularly if you intend to handhold your camera and lens combination for extended periods. Sigma 18-50mm F2.8-4.5 DC OS HSM weighs 395g, which means it is 90g (18%) lighter than the Pentax 17-70mm F4.0 AL SDM which has a weight of 485g.
Below you can find the dimension and weight table of the Standard Zoom Lenses in Pentax KAF3 Mount for size comparison purposes.
Standard Lenses in Pentax KAF3 Mount for Size Comparison
Filter Threads
Both the Sigma 18-50mm F2.8-4.5 DC OS HSM and the Pentax 17-70mm F4.0 AL SDM have the same Filter thread size of 67mm.
Below are links to the 67mm filters that we recommend you to consider for these lenses:
Lens Mounts
Both the Sigma 18-50mm F2.8-4.5 DC OS HSM and the Pentax 17-70mm F4.0 AL SDM has the same Pentax KAF3 lens mount. Some of the latest released cameras that are compatible with the Pentax KAF3 lenses are , and .
Sigma 18-50mm F2.8-4.5 DC OS HSM is also available in the Canon EF , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
Sigma 18-50mm F2.8-4.5 DC OS HSM has a focal range of 18-50mm and
2.8X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of
27-75mm when used on a APS-C / DX format camera.
On the other hand, the Pentax 17-70mm F4.0 AL SDM 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.
