The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 24-70mm F2.8 EX DG 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 24-70mm F2.8 EX DG is the longer of the two lenses at 116mm. The Pentax 18-55mm F3.5-5.6 AL with a length of 68mm, is 48mm shorter. Besides being longer, the Sigma 24-70mm F2.8 EX DG also has a larger diameter of 89mm 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, 68% (490g) lighter than the Sigma 24-70mm F2.8 EX DG's weight of 715g.
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 24-70mm F2.8 EX DG has a filter size of 82mm 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 82mm filters that we recommend you to consider for the Sigma 24-70mm F2.8 EX DG:
And these are the 52mm filters that we recommend for the Pentax 18-55mm F3.5-5.6 AL:
Lens Mounts
Both the Sigma 24-70mm F2.8 EX DG 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 24-70mm F2.8 EX DG is also available in Canon EF , Sony Alpha, Sigma SA and Nikon F (FX) mounts.
Focal Range
Sigma 24-70mm F2.8 EX DG has a focal range of 24-70mm and
2.9X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
36 - 105mm.
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.