The type of photography that you are planning to use is an important decision factor. Here we have evaluated the HD Pentax-FA 31mm F1.8 Ltd 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.HD Pentax-FA 31mm F1.8 Ltd is the longer of the two lenses at 69mm. The Pentax 21mm F3.2 AL L with a length of 25mm is 44mm shorter. Besides being longer, the HD Pentax-FA 31mm F1.8 Ltd also has a larger diameter of 65mm 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 140g, 58% (201g) lighter than the HD Pentax-FA 31mm F1.8 Ltd's weight of 341g.
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 HD Pentax-FA 31mm F1.8 Ltd 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.
Pentax 21mm F3.2 AL L is also available in Pentax KAF mount.
Focal Range
HD Pentax-FA 31mm F1.8 Ltd is a prime lens with fixed focal lenght of 31mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides an 35mm (FF) equivalent of 46.5mm.
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.