The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Venus 25mm F2.8 2.5-5X Ultra Macro 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. Venus 25mm F2.8 2.5-5X Ultra Macro is the longer of the two lenses at 65mm. The Pentax 21mm F3.2 AL L with a length of 25mm, is 40mm shorter. Besides being longer, the Venus 25mm F2.8 2.5-5X Ultra Macro also has a larger diameter of 82mm 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, 66% (266g) lighter than the Venus 25mm F2.8 2.5-5X Ultra Macro's weight of 400g.
Lens Mounts
The Venus 25mm F2.8 2.5-5X Ultra Macro has the Pentax KAF2 lens mount whereas the Pentax 21mm F3.2 AL L has the Pentax KAF lens mount. Some of the latest released cameras that are compatible with these mounts are Pentax K-3 III Monochrome, Pentax KF and Pentax K-3 III for the Pentax KAF2 Mount and Pentax K-3 III Monochrome, Pentax KF and Pentax K-3 III for the Pentax KAF Mount.
Venus 25mm F2.8 2.5-5X Ultra Macro is also available in Canon EF , Nikon F (DX) and Sony E mounts.
Pentax 21mm F3.2 AL L is also available in Pentax KAF mount.
Focal Range
Venus 25mm F2.8 2.5-5X Ultra Macro is a prime lens with fixed focal lenght of 25mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
37.5mm.
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.