The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 24mm F1.8 EX DG and the Sigma 20mm F1.8 EX DG RF for their suitability to various photography scenarios:
Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Sigma 20mm F1.8 EX DG RF is the longer of the two lenses at 90mm. The Sigma 24mm F1.8 EX DG with a length of 83mm is 7mm shorter. Besides being longer, the Sigma 20mm F1.8 EX DG RF also has a larger diameter of 89mm compared to Sigma 24mm F1.8 EX DG's 84mm diameter.
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 24mm F1.8 EX DG weighs 485g, which means it is 35g (6%) lighter than the Sigma 20mm F1.8 EX DG RF which has a weight of 520g.
Below you can find the dimension and weight table of the 'Wideangle Prime Lenses in Pentax KAF Mount for size comparison purposes.
Wideangle Prime Lenses in Pentax KAF Mount for Size Comparison
Lens Mounts
Both Sigma 24mm F1.8 EX DG and Sigma 20mm F1.8 EX DG RF has the same Pentax KAF lens mount. Some of the latest released cameras that are compatible with the Pentax KAF lenses are Pentax KF, Pentax K-3 III and Pentax K-1 II.
Sigma 24mm F1.8 EX DG is also available in Canon EF , Nikon F (FX), Four Thirds, Sony Alpha, Sigma SA, Canon EF, Pentax KAF, Sony Alpha, Sigma SA and Nikon F (FX) mounts.
Sigma 20mm F1.8 EX DG RF is also available in Canon EF , Nikon F (DX), Sony Alpha and Sigma SA mounts.
Focal Range
Sigma 24mm F1.8 EX DG 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.