The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 20mm F1.4 DG HSM A and the Tamron 17-50mm f2.8 XR II LD 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 F1.4 DG HSM A is the longer of the two lenses at 130mm. The Tamron 17-50mm f2.8 XR II LD with a length of 82mm, is 48mm shorter. Besides being longer, the Sigma 20mm F1.4 DG HSM A also has a larger diameter of 91mm compared to the Tamron 17-50mm f2.8 XR II LD'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. Tamron 17-50mm f2.8 XR II LD weighs 430g, 54% (520g) lighter than the Sigma 20mm F1.4 DG HSM A's weight of 950g.
Lens Mounts
Both the Sigma 20mm F1.4 DG HSM A and the Tamron 17-50mm f2.8 XR II LD has the same Canon EF lens mount. Some of the latest released cameras that are compatible with the Canon EF lenses are Canon 1D X III, Canon 6D MII and Canon 5D MIV.
Sigma 20mm F1.4 DG HSM A is also available in Nikon F (FX) , Sigma SA and Leica L mounts.
Tamron 17-50mm f2.8 XR II LD is also available in Nikon F (DX) , Sony Alpha and Pentax KAF mounts.
Focal Range
Sigma 20mm F1.4 DG HSM A is a prime lens with fixed focal lenght of 20mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of
32mm.
On the other hand, the Tamron 17-50mm f2.8 XR II LD has a focal range of 17-50mm and 2.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27.2-80mm when used on a APS-C / DX format camera.