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 Samyang 20mm F1.8 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. Samyang 20mm F1.8 is the longer of the two lenses at 88mm. The Pentax 21mm F3.2 AL L with a length of 25mm, is 63mm shorter. Besides being longer, the Samyang 20mm F1.8 also has a larger diameter of 83mm 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 134g, which means it is 363g (73%) lighter than the Samyang 20mm F1.8 which has a weight of 497g.
Filter Threads
The Pentax 21mm F3.2 AL L has a filter size of 49mm whereas the Samyang 20mm F1.8 has a 77mm 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 77mm filters that we recommend for the Samyang 20mm F1.8:
Lens Mounts
Both the Pentax 21mm F3.2 AL L and the Samyang 20mm F1.8 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.
Samyang 20mm F1.8 is also available in Canon EF , Canon EF-M, Fujifilm X, Micro Four Thirds, Nikon F (FX), Samsung NX, Sony Alpha and Sony E 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 Samyang 20mm F1.8 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.