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 DA 20-40mm F2.8-4 ED DC WR for their suitability to various photography scenarios:
Size and Weight Comparison
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 DA 20-40mm F2.8-4 ED DC WR weighs 283g, 17% (58g) lighter than the HD Pentax-FA 31mm F1.8 Ltd's weight of 341g.
Filter Threads
The HD Pentax-FA 31mm F1.8 Ltd has a filter size of 58mm whereas the Pentax DA 20-40mm F2.8-4 ED DC WR has a 55mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 58mm filters that we recommend you to consider for the HD Pentax-FA 31mm F1.8 Ltd:
And these are the 55mm filters that we recommend for the Pentax DA 20-40mm F2.8-4 ED DC WR:
Lens Mounts
The HD Pentax-FA 31mm F1.8 Ltd has the Pentax KAF lens mount whereas the Pentax DA 20-40mm F2.8-4 ED DC WR has the Pentax KAF3 lens mount. Some of the latest released cameras that are compatible with these mounts are Pentax K-3 III Mono, Pentax KF and Pentax K-3 III for the Pentax KAF Mount and , and for the Pentax KAF3 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 a 35mm (FF) equivalent of
46.5mm.
On the other hand, the Pentax DA 20-40mm F2.8-4 ED DC WR has a focal range of 20-40mm and 2.0X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 30-60mm when used on a APS-C / DX format camera.
