The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Pentax 15mm F4 L and the Pentax 21mm F3.2 AL L 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. Pentax 15mm F4 L is the longer of the two lenses at 40mm. The Pentax 21mm F3.2 AL L with a length of 25mm, is 15mm shorter. On the other hand, there is no difference between the max diameters of these lenses, both coming at 63mm.
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, 36% (78g) lighter than the Pentax 15mm F4 L's weight of 212g.
Below you can find the dimension and weight table of the Wideangle Prime Lenses in Pentax KAF Mount for size comparison purposes.
Wideangle Lenses in Pentax KAF Mount for Size Comparison
Filter Threads
Both the Pentax 15mm F4 L and the Pentax 21mm F3.2 AL L have the same Filter thread size of 49mm.
Below are links to the 49mm filters that we recommend you to consider for these lenses:
Lens Mounts
Both the Pentax 15mm F4 L and the 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 K-3 III Monochrome, Pentax KF and Pentax K-3 III.
Pentax 21mm F3.2 AL L is also available in Pentax KAF mount.
Focal Range
Pentax 15mm F4 L is a prime lens with fixed focal lenght of 15mm which has an effective (full-frame 35mm equivalent) focal range of
22.5mm when used on a APS-C / DX format camera.
On the other hand, the 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.