The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 24mm F1.4 DG HSM A and the Sigma 20mm F1.8 EX DG RF for their suitability to various photography scenarios:
Size and Weight Comparison
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. Sigma 20mm F1.8 EX DG RF weighs 520g, 21% (145g) lighter than the Sigma 24mm F1.4 DG HSM A's weight of 665g.
Lens Mounts
Both Sigma 24mm F1.4 DG HSM A has the Nikon F (FX) lens mount whereas the Sigma 20mm F1.8 EX DG RF has the Nikon F (DX) lens mount. Some of the latest released cameras that are compatible with these mounts are Nikon D6, Nikon D780 and Nikon D3500 for the Nikon F (FX) Mount and Nikon D6, Nikon D780 and Nikon D3500 for the Nikon F (DX) Mount.
Sigma 24mm F1.4 DG HSM A is also available in Canon EF , Sigma SA and Leica L mounts.
Sigma 20mm F1.8 EX DG RF is also available in Canon EF , Pentax KAF, Sony Alpha and Sigma SA mounts.
Focal Range
Sigma 24mm F1.4 DG HSM A is a prime lens with fixed focal lenght of 24mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides an 35mm (FF) equivalent of 36mm.
Sigma 20mm F1.8 EX DG RF is a prime lens with fixed focal lenght of 20mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides an 35mm (FF) equivalent of 30mm.