The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 120-400mm F4.5-5.6 DG OS HSM and the Tamron 100-400mm F4.5-6.3 Di VC USD 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. Sigma 120-400mm F4.5-5.6 DG OS HSM is the longer of the two lenses at 203mm. The Tamron 100-400mm F4.5-6.3 Di VC USD with a length of 199mm, is 4mm shorter. Besides being longer, the Sigma 120-400mm F4.5-5.6 DG OS HSM also has a larger diameter of 92mm compared to the Tamron 100-400mm F4.5-6.3 Di VC USD's 86mm 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. Tamron 100-400mm F4.5-6.3 Di VC USD weighs 1135g, 30% (505g) lighter than the Sigma 120-400mm F4.5-5.6 DG OS HSM's weight of 1640g.
Below you can find the dimension and weight table of the Telephoto Zoom Lenses in Canon EF Mount for size comparison purposes.
Super Telephoto Lenses in Canon EF Mount for Size Comparison
Filter Threads
The Sigma 120-400mm F4.5-5.6 DG OS HSM has a filter size of 77mm whereas the Tamron 100-400mm F4.5-6.3 Di VC USD 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 77mm filters that we recommend you to consider for the Sigma 120-400mm F4.5-5.6 DG OS HSM:
And these are the 67mm filters that we recommend for the Tamron 100-400mm F4.5-6.3 Di VC USD:
Lens Mounts
Both the Sigma 120-400mm F4.5-5.6 DG OS HSM and the Tamron 100-400mm F4.5-6.3 Di VC USD 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.
Sigma 120-400mm F4.5-5.6 DG OS HSM is also available in Sigma SA and Nikon F (FX) mounts.
Tamron 100-400mm F4.5-6.3 Di VC USD is also available in Nikon F (FX) mount.
Focal Range
Sigma 120-400mm F4.5-5.6 DG OS HSM has a focal range of 120-400mm and
3.3X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of
192 - 640mm.
On the other hand, the Tamron 100-400mm F4.5-6.3 Di VC USD has a focal range of 100-400mm and 4.0X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of 160 - 640mm.