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
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 20mm F1.8 EX DG RF weighs 520g, 21% (145g) lighter than the Sigma 24mm F1.4 DG HSM A's weight of 665g.
Filter Threads
The Sigma 24mm F1.4 DG HSM A has a filter size of 77mm whereas the Sigma 20mm F1.8 EX DG RF has a 82mm 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 24mm F1.4 DG HSM A:
And these are the 82mm filters that we recommend for the Sigma 20mm F1.8 EX DG RF:
Lens Mounts
The 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 a 35mm (FF) equivalent of
36mm.
On the other hand, the 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 a 35mm (FF) equivalent of 30mm.