The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 20mm F1.8 EX DG RF and the Pentax 21mm F3.2 AL L 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 Pentax 21mm F3.2 AL L with a length of 25mm is 65mm shorter. Besides being longer, the Sigma 20mm F1.8 EX DG RF also has a larger diameter of 89mm compared to Pentax 21mm F3.2 AL L's 63mm 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. Pentax 21mm F3.2 AL L weighs 134g, 74% (386g) lighter than the Sigma 20mm F1.8 EX DG RF's 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
Both Sigma 20mm F1.8 EX DG RF and Pentax 21mm F3.2 AL L 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 20mm F1.8 EX DG RF is also available in Canon EF , Nikon F (DX), Sony Alpha and Sigma SA mounts.
Pentax 21mm F3.2 AL L is also available in Pentax KAF mount.
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.
Pentax 21mm F3.2 AL L is a prime lens with fixed focal lenght of 21mm which has an effective (full-frame 35mm equivalent) focal range of 31.5mm when used on a APS-C / DX format camera.