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 18-50mm F2.8-4.5 DC OS HSM for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is a crucial factor to consider when comparing two lenses. Tamron 24-70mm f2.8 VC USD is the longer of the two lenses at 117mm. The Sigma 18-50mm F2.8-4.5 DC OS HSM with a length of 89mm, is 28mm shorter. Besides being longer, the Tamron 24-70mm f2.8 VC USD also has a larger diameter of 88mm compared to the Sigma 18-50mm F2.8-4.5 DC OS HSM's 74mm diameter.
The weight of a lens is equally significant as its external dimensions, particularly if you intend to handhold your camera and lens combination for extended periods. Sigma 18-50mm F2.8-4.5 DC OS HSM weighs 395g, 52% (430g) 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 Sony Alpha Mount for size comparison purposes.
Standard Lenses in Sony Alpha Mount for Size Comparison
Filter Threads
The Tamron 24-70mm f2.8 VC USD has a filter size of 82mm whereas the Sigma 18-50mm F2.8-4.5 DC OS HSM has a 67mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 82mm filters that we recommend you to consider for the Tamron 24-70mm f2.8 VC USD:
And these are the 67mm filters that we recommend for the Sigma 18-50mm F2.8-4.5 DC OS HSM:
Lens Mounts
Both the Tamron 24-70mm f2.8 VC USD and the 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 24-70mm f2.8 VC USD is also available in Canon EF and Nikon F (FX) 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 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 1.5x crop, it provides a 35mm (FF) equivalent of
36 - 105mm.
On the other hand, the 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.