The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Tamron AF 28-75mm f2.8 XR LD and the Sigma 18-35mm F1.8 DC HSM for their suitability to various photography scenarios:
Size and Weight Comparison
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 Tamron AF 28-75mm f2.8 XR LD with a length of 92mm is 29mm shorter. Besides being longer, the Sigma 18-35mm F1.8 DC HSM also has a larger diameter of 78mm compared to Tamron AF 28-75mm f2.8 XR LD's 73mm diameter.
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. Tamron AF 28-75mm f2.8 XR LD weighs 510g, which means it is 300g (37%) 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
Both Tamron AF 28-75mm f2.8 XR LD and 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.
Tamron AF 28-75mm f2.8 XR LD is also available in Canon EF , Nikon F (FX) and Pentax KAF mounts.
Sigma 18-35mm F1.8 DC HSM is also available in Canon EF , Pentax KAF, Sigma SA and Nikon F (DX) mounts.
Tamron AF 28-75mm f2.8 XR LD 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 an 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.