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 18-50mm F2.8-4.5 DC OS 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.Tamron AF 28-75mm f2.8 XR LD is the longer of the two lenses at 92mm. The Sigma 18-50mm F2.8-4.5 DC OS HSM with a length of 89mm is 3mm shorter. On the other hand, the Sigma 18-50mm F2.8-4.5 DC OS HSM has a larger diameter of 74mm 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. Sigma 18-50mm F2.8-4.5 DC OS HSM weighs 395g, 22% (115g) lighter than the Tamron AF 28-75mm f2.8 XR LD's weight of 510g.
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 Tamron AF 28-75mm f2.8 XR LD and Sigma 18-50mm F2.8-4.5 DC OS 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 18-50mm F2.8-4.5 DC OS HSM is also available in Canon EF , Pentax KAF3, Sigma SA and Nikon F (DX) mounts.
Focal Range
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 18-50mm F2.8-4.5 DC OS HSM has a focal range of 18-50mm and 2.8X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-75mm when used on a APS-C / DX format camera.