The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 800mm F5.6 EX DG HSM and the Canon 400mm f2.8L IS II USM for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is an important deciding factor when comparing two lenses. Sigma 800mm F5.6 EX DG HSM is the longer of the two lenses at 521mm. The Canon 400mm f2.8L IS II USM with a length of 343mm, is 178mm shorter. On the other hand, the Canon 400mm f2.8L IS II USM has a larger diameter of 163mm compared to the Sigma 800mm F5.6 EX DG HSM's 157mm diameter.
The 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. Canon 400mm f2.8L IS II USM weighs 3850g, 21% (1050g) lighter than the Sigma 800mm F5.6 EX DG HSM's weight of 4900g.
Below you can find the dimension and weight table of the Telephoto Prime Lenses in Canon EF Mount for size comparison purposes.
Telephoto Prime Lenses in Canon EF Mount for Size Comparison
Lens Mounts
Both the Sigma 800mm F5.6 EX DG HSM and the Canon 400mm f2.8L IS II USM 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 800mm F5.6 EX DG HSM is also available in Nikon F (FX) and Sigma SA mounts.
Focal Range
Sigma 800mm F5.6 EX DG HSM is a prime lens with fixed focal lenght of 800mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of 1280mm.
Canon 400mm f2.8L IS II USM is a prime lens with fixed focal lenght of 400mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of 640mm.