Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Sigma 24-70mm F2.8 EX DG is the longer of the two lenses at 116mm. The Tamron AF 28-75mm f2.8 XR LD with a length of 92mm is 24mm shorter. Besides being longer, the Sigma 24-70mm F2.8 EX DG also has a larger diameter of 89mm 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 205g (40%) lighter than the Sigma 24-70mm F2.8 EX DG which has a 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 Zoom Lenses in Pentax KAF Mount for Size Comparison
Lens Mounts
Both Tamron AF 28-75mm f2.8 XR LD and Sigma 24-70mm F2.8 EX DG 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, Pentax K-1 II and Pentax KP.
Tamron AF 28-75mm f2.8 XR LD is also available in Canon EF , Nikon F (FX) and Sony Alpha mounts.
Sigma 24-70mm F2.8 EX DG is also available in Canon EF , Sony Alpha, Sigma SA and Nikon F (FX) 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 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 an 35mm (FF) equivalent of 36 - 105mm.