The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon 100-300mm f4.5-5.6 USM and the Sigma 120-300mm F2.8 EX DG 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. Sigma 120-300mm F2.8 EX DG HSM is the longer of the two lenses at 271mm. The Canon 100-300mm f4.5-5.6 USM with a length of 122mm, is 149mm shorter. Besides being longer, the Sigma 120-300mm F2.8 EX DG HSM also has a larger diameter of 113mm compared to the Canon 100-300mm f4.5-5.6 USM's 73mm 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. Canon 100-300mm f4.5-5.6 USM weighs 540g, which means it is 2060g (79%) lighter than the Sigma 120-300mm F2.8 EX DG HSM which has a weight of 2600g.
Below you can find the dimension and weight table of the Telephoto Zoom Lenses in Canon EF Mount for size comparison purposes.
Telephoto Lenses in Canon EF Mount for Size Comparison
Filter Threads
The Canon 100-300mm f4.5-5.6 USM has a filter size of 58mm whereas the Sigma 120-300mm F2.8 EX DG HSM has a 105mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 58mm filters that we recommend you to consider for the Canon 100-300mm f4.5-5.6 USM:
And these are the 105mm filters that we recommend for the Sigma 120-300mm F2.8 EX DG HSM:
Lens Mounts
Both the Canon 100-300mm f4.5-5.6 USM and the Sigma 120-300mm F2.8 EX DG 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.
Sigma 120-300mm F2.8 EX DG HSM is also available in Sigma SA and Nikon F (FX) mounts.
Focal Range
Canon 100-300mm f4.5-5.6 USM has a focal range of 100-300mm and
3.0X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of
160 - 480mm.
On the other hand, the Sigma 120-300mm F2.8 EX DG HSM has a focal range of 120-300mm and 2.5X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of 192 - 480mm.