Prior to the release of the Canon EF 11-24mm ƒ/4L lens, the widest full-frame rectilinear lens Canon offered was a 14mm ƒ/2.8L II prime lens; and for a zoom, it's a pair of 16-35mm L zoom lenses.
The Sigma has less vignetting. Looking at the specs and measurements, the Canon RF 50mm F1.2 L USM Lens vs. Sigma 50mm f/1.4 DG HSM Art Lens comparison shows these two lenses being rather similar. The Sigma is slightly lighter and smaller, but not weather sealed. The Canon has one additional aperture blade, 10 vs. 9. You can also use this lens with peace of mind if you want to take backlit shots that include the sun in the frame. EOS 5D Mark III/ EF24-70mm f/2.8L II USM/ FL: 24mm/ Manual exposure (f/8, 1/2 sec)/ ISO 100/ WB: Daylight. This shot of a maple tree with autumn leaves was taken in light rain. While I was shooting, the rain suddenly became heavy . 24 69 105 20 333 355 187 99 5