The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sony 28-75mm F2.8 SAM and the Sigma 18-35mm F1.8 DC HSM for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is an important deciding factor when comparing two lenses. Sigma 18-35mm F1.8 DC HSM is the longer of the two lenses at 121mm. The Sony 28-75mm F2.8 SAM with a length of 98mm, is 23mm shorter. On the other hand, there is no difference between the max diameters of these lenses, both coming at 78mm.
The 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. Sony 28-75mm F2.8 SAM weighs 565g, which means it is 245g (30%) lighter than the Sigma 18-35mm F1.8 DC HSM which has a weight of 810g.
Below you can find the dimension and weight table of the Standard Zoom Lenses in Sony Alpha Mount for size comparison purposes.
Standard Zoom Lenses in Sony Alpha Mount for Size Comparison
Lens Mounts
Both the Sony 28-75mm F2.8 SAM and the Sigma 18-35mm F1.8 DC HSM has the same Sony Alpha lens mount. Some of the latest released cameras that are compatible with the Sony Alpha lenses are Sony A99 II, Sony A68 and Sony A77 II.
Sigma 18-35mm F1.8 DC HSM is also available in Canon EF , Pentax KAF, Sigma SA and Nikon F (DX) mounts.
Focal Range
Sony 28-75mm F2.8 SAM has a focal range of 28-75mm and 2.7X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of 42 - 112.5mm.
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.