The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Pentax 21mm F3.2 AL L and the Sigma 20mm F1.8 EX DG RF for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is a crucial factor to consider 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 the Pentax 21mm F3.2 AL L's 63mm diameter.
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. Pentax 21mm F3.2 AL L weighs 140g, which means it is 380g (73%) lighter than the Sigma 20mm F1.8 EX DG RF which has a weight of 520g.
Filter Threads
The Pentax 21mm F3.2 AL L has a filter size of 49mm 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 49mm filters that we recommend you to consider for the Pentax 21mm F3.2 AL L:
And these are the 82mm filters that we recommend for the Sigma 20mm F1.8 EX DG RF:
Lens Mounts
Both the Pentax 21mm F3.2 AL L and the 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 K-3 III Monochrome, Pentax KF and Pentax K-3 III.
Pentax 21mm F3.2 AL L is also available in Pentax KAF mount.
Sigma 20mm F1.8 EX DG RF is also available in Canon EF , Nikon F (DX), Sony Alpha and Sigma SA mounts.
Focal Range
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.
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.