The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Canon 20mm f2.8 USM and the Tamron 17-50mm f2.8 XR II VC 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. Tamron 17-50mm f2.8 XR II VC LD is the longer of the two lenses at 95mm. The Canon 20mm f2.8 USM with a length of 71mm, is 24mm shorter. Besides being longer, the Tamron 17-50mm f2.8 XR II VC LD also has a larger diameter of 80mm compared to the Canon 20mm f2.8 USM's 78mm 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. Canon 20mm f2.8 USM weighs 405g, which means it is 165g (28%) lighter than the Tamron 17-50mm f2.8 XR II VC LD which has a weight of 570g.
Filter Threads
Both the Canon 20mm f2.8 USM and the Tamron 17-50mm f2.8 XR II VC LD have the same Filter thread size of 72mm.
Below are links to the 72mm filters that we recommend you to consider for these lenses:
Lens Mounts
Both the Canon 20mm f2.8 USM and the Tamron 17-50mm f2.8 XR II VC 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.
Tamron 17-50mm f2.8 XR II VC LD is also available in Nikon F (DX) mount.
Focal Range
Canon 20mm f2.8 USM 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 VC 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.