The type of photography that you are planning to use is an important decision factor. Here we have evaluated the HD Pentax-DA 10-17mm F3.5-4.5 ED fisheye and the Sigma 20mm F1.8 EX DG RF 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. Sigma 20mm F1.8 EX DG RF is the longer of the two lenses at 90mm. The HD Pentax-DA 10-17mm F3.5-4.5 ED fisheye with a length of 70mm, is 20mm shorter. Besides being longer, the Sigma 20mm F1.8 EX DG RF also has a larger diameter of 89mm compared to the HD Pentax-DA 10-17mm F3.5-4.5 ED fisheye's 68mm 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. HD Pentax-DA 10-17mm F3.5-4.5 ED fisheye weighs 317g, which means it is 203g (39%) lighter than the Sigma 20mm F1.8 EX DG RF which has a weight of 520g.
Lens Mounts
Both the HD Pentax-DA 10-17mm F3.5-4.5 ED fisheye and the Sigma 20mm F1.8 EX DG RF 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 Mono, Pentax KF and Pentax K-3 III.
Sigma 20mm F1.8 EX DG RF is also available in Canon EF , Nikon F (DX), Sony Alpha and Sigma SA mounts.
Focal Range
HD Pentax-DA 10-17mm F3.5-4.5 ED fisheye has a focal range of 10-17mm and
1.7X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of
15-25.5mm when used on a APS-C / DX format camera.
On the other hand, the Sigma 20mm F1.8 EX DG RF 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.
