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 Sigma 100-400mm F5-6.3 DG 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. Sigma 120-400mm F4.5-5.6 DG OS HSM is the longer of the two lenses at 203mm. The Sigma 100-400mm F5-6.3 DG OS HSM with a length of 182mm, is 21mm 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 Sigma 100-400mm F5-6.3 DG OS HSM'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. Sigma 100-400mm F5-6.3 DG OS HSM weighs 1160g, 29% (480g) 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 Nikon F (FX) Mount for size comparison purposes.
Super Telephoto Lenses in Nikon F (FX) 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 Sigma 100-400mm F5-6.3 DG 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 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 Sigma 100-400mm F5-6.3 DG OS HSM:
Lens Mounts
Both the Sigma 120-400mm F4.5-5.6 DG OS HSM and the Sigma 100-400mm F5-6.3 DG OS HSM has the same Nikon F (FX) lens mount. Some of the latest released cameras that are compatible with the Nikon F (FX) lenses are Nikon D6, Nikon D780 and Nikon D3500.
Sigma 120-400mm F4.5-5.6 DG OS HSM is also available in Canon EF and Sigma SA mounts.
Sigma 100-400mm F5-6.3 DG OS HSM is also available in Sigma SA and Canon EF mounts.
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 1.5x crop, it provides a 35mm (FF) equivalent of
180 - 600mm.
On the other hand, the Sigma 100-400mm F5-6.3 DG OS HSM has a focal range of 100-400mm and 4.0X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of 150 - 600mm.