The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 18-35mm F1.8 DC HSM and the Pentax 18-55mm F3.5-5.6 AL 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 18-35mm F1.8 DC HSM is the longer of the two lenses at 121mm. The Pentax 18-55mm F3.5-5.6 AL with a length of 68mm, is 53mm shorter. Besides being longer, the Sigma 18-35mm F1.8 DC HSM also has a larger diameter of 78mm compared to the Pentax 18-55mm F3.5-5.6 AL'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 AL weighs 225g, 72% (585g) lighter than the Sigma 18-35mm F1.8 DC HSM's weight of 810g.
Below you can find the dimension and weight table of the Standard Zoom Lenses in Pentax KAF Mount for size comparison purposes.
Standard Lenses in Pentax KAF Mount for Size Comparison
Filter Threads
The Sigma 18-35mm F1.8 DC HSM has a filter size of 72mm whereas the Pentax 18-55mm F3.5-5.6 AL 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 72mm filters that we recommend you to consider for the Sigma 18-35mm F1.8 DC HSM:
And these are the 52mm filters that we recommend for the Pentax 18-55mm F3.5-5.6 AL:
Lens Mounts
Both the Sigma 18-35mm F1.8 DC HSM and the Pentax 18-55mm F3.5-5.6 AL has the same Pentax KAF lens mount. Some of the latest released cameras that are compatible with the Pentax KAF lenses are Pentax K-3 III Monochrome, Pentax KF and Pentax K-3 III.
Sigma 18-35mm F1.8 DC HSM is also available in Canon EF , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
Sigma 18-35mm F1.8 DC HSM has a focal range of 18-35mm and
1.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of
27-52.5mm when used on a APS-C / DX format camera.
On the other hand, the Pentax 18-55mm F3.5-5.6 AL 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.