The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 17-50mm F2.8 EX DC OS HSM and the Pentax 18-55mm F3.5-5.6 ED II 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. Sigma 17-50mm F2.8 EX DC OS HSM is the longer of the two lenses at 92mm. The Pentax 18-55mm F3.5-5.6 ED II with a length of 68mm, is 24mm shorter. Besides being longer, the Sigma 17-50mm F2.8 EX DC OS HSM also has a larger diameter of 84mm compared to the Pentax 18-55mm F3.5-5.6 ED II's 68mm 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. Pentax 18-55mm F3.5-5.6 ED II weighs 220g, 61% (345g) lighter than the Sigma 17-50mm F2.8 EX DC OS HSM's weight of 565g.
Filter Threads
The Sigma 17-50mm F2.8 EX DC OS HSM has a filter size of 77mm whereas the Pentax 18-55mm F3.5-5.6 ED II has a 52mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 77mm filters that we recommend you to consider for the Sigma 17-50mm F2.8 EX DC OS HSM:
And these are the 52mm filters that we recommend for the Pentax 18-55mm F3.5-5.6 ED II:
Lens Mounts
The Sigma 17-50mm F2.8 EX DC OS HSM has the Pentax KAF3 lens mount whereas the Pentax 18-55mm F3.5-5.6 ED II has the Pentax KAF lens mount. Some of the latest released cameras that are compatible with these mounts are , and for the Pentax KAF3 Mount and Pentax K-3 III Monochrome, Pentax KF and Pentax K-3 III for the Pentax KAF Mount.
Sigma 17-50mm F2.8 EX DC OS HSM is also available in Canon EF , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
Sigma 17-50mm F2.8 EX DC OS HSM has a focal range of 17-50mm and
2.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of
25.5-75mm when used on a APS-C / DX format camera.
On the other hand, the Pentax 18-55mm F3.5-5.6 ED II has a focal range of 18-55mm and 3.1X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-82.5mm when used on a APS-C / DX format camera.