The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Samyang 20mm F1.8 and the Sigma 15mm F2.8 EX DG 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. Samyang 20mm F1.8 is the longer of the two lenses at 88mm. The Sigma 15mm F2.8 EX DG Fisheye with a length of 65mm, is 23mm shorter. Besides being longer, the Samyang 20mm F1.8 also has a larger diameter of 83mm compared to the Sigma 15mm F2.8 EX DG Fisheye's 74mm 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. Sigma 15mm F2.8 EX DG Fisheye weighs 370g, 25% (127g) lighter than the Samyang 20mm F1.8's weight of 497g.
Below you can find the dimension and weight table of the Wideangle Prime Lenses in Sony Alpha Mount for size comparison purposes.
Ultra-Wideangle Lenses in Sony Alpha Mount for Size Comparison
Lens Mounts
Both the Samyang 20mm F1.8 and the Sigma 15mm F2.8 EX DG Fisheye has the same Sony Alpha lens mount. Some of the latest released cameras that are compatible with the Sony Alpha lenses are Sony A99 II, Sony A68 and Sony A77 II.
Samyang 20mm F1.8 is also available in the Canon EF , Canon EF-M, Fujifilm X, Micro Four Thirds, Nikon F (FX), Pentax KAF, Samsung NX and Sony E mounts.
Sigma 15mm F2.8 EX DG Fisheye is also available in Canon EF , Nikon F (FX), Pentax KAF and Sigma SA mounts.
Focal Range
Samyang 20mm F1.8 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 Sigma 15mm F2.8 EX DG Fisheye is a prime lens with fixed focal lenght of 15mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of 22.5mm.
