The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Tamron 24-70mm f2.8 VC USD 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 24-70mm f2.8 VC USD with a length of 117mm is 4mm shorter. On the other hand, the Tamron 24-70mm f2.8 VC USD has a larger diameter of 88mm compared to Sigma 18-35mm F1.8 DC HSM's 78mm 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 18-35mm F1.8 DC HSM weighs 810g, 1% (15g) lighter than the Tamron 24-70mm f2.8 VC USD's weight of 825g.
Below you can find the dimension and weight table of the 'Standard Zoom Lenses in Canon EF Mount for size comparison purposes.
Standard Zoom Lenses in Canon EF Mount for Size Comparison
Lens Mounts
Both Tamron 24-70mm f2.8 VC USD and Sigma 18-35mm F1.8 DC HSM has the same Canon EF lens mount. Some of the latest released cameras that are compatible with the Canon EF lenses are Canon 1D X III, Canon 6D MII and Canon 5D MIV.
Tamron 24-70mm f2.8 VC USD is also available in Nikon F (FX) and Sony Alpha mounts.
Sigma 18-35mm F1.8 DC HSM is also available in Pentax KAF , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
Tamron 24-70mm f2.8 VC USD has a focal range of 24-70mm and 2.9X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides an 35mm (FF) equivalent of 38.4 - 112mm.
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 28.8-56mm when used on a APS-C / DX format camera.