The type of photography that you are planning to use is an important decision factor. Here we have evaluated the ZEISS Loxia 21mm F2.8 and the Sigma 19mm F2.8 DN 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. Sigma 19mm F2.8 DN weighs 140g, 64% (254g) lighter than the ZEISS Loxia 21mm F2.8's weight of 394g.
Below you can find the dimension and weight table of the Wideangle Prime Lenses in Sony E Mount for size comparison purposes.
Wideangle Lenses in Sony E Mount for Size Comparison
Lens Mounts
Both the ZEISS Loxia 21mm F2.8 and the Sigma 19mm F2.8 DN has the same Sony E lens mount. Some of the latest released cameras that are compatible with the Sony E lenses are Sony A9 III, Sony A7C Mark II and Sony A7CR.
Sigma 19mm F2.8 DN is also available in Micro Four Thirds mount.
Focal Range
ZEISS Loxia 21mm F2.8 is a prime lens with fixed focal lenght of 21mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
31.5mm.
On the other hand, the Sigma 19mm F2.8 DN is a prime lens with fixed focal lenght of 19mm which has an effective (full-frame 35mm equivalent) focal range of 28.5mm when used on a APS-C / DX format camera.