The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 20mm F1.8 EX DG RF and the Rokinon 8mm f3.5 Fisheye 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 Rokinon 8mm f3.5 Fisheye with a length of 77mm, is 13mm shorter. Besides being longer, the Sigma 20mm F1.8 EX DG RF also has a larger diameter of 89mm compared to the Rokinon 8mm f3.5 Fisheye's 75mm 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. Rokinon 8mm f3.5 Fisheye weighs 443g, 14% (77g) lighter than the Sigma 20mm F1.8 EX DG RF's weight of 520g.
Below you can find the dimension and weight table of the Wideangle Prime Lenses in Pentax KAF Mount for size comparison purposes.
Ultra-Wideangle Lenses in Pentax KAF Mount for Size Comparison
Lens Mounts
Both the Sigma 20mm F1.8 EX DG RF and the Rokinon 8mm f3.5 Fisheye 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 the Canon EF , Nikon F (DX), Sony Alpha and Sigma SA mounts.
Rokinon 8mm f3.5 Fisheye is also available in Canon EF , Nikon F (FX), Four Thirds, Sony Alpha and Samsung NX mounts.
Focal Range
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.
On the other hand, the Rokinon 8mm f3.5 Fisheye is a prime lens with fixed focal lenght of 8mm which has an effective (full-frame 35mm equivalent) focal range of 12mm when used on a APS-C / DX format camera.
