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 18-270mm f3.5-6.3 II VC PZD 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 18-270mm f3.5-6.3 II VC PZD is the longer of the two lenses at 88mm. The Canon 20mm f2.8 USM with a length of 71mm, is 17mm shorter. On the other hand, the Canon 20mm f2.8 USM has a larger diameter of 78mm compared to the Tamron 18-270mm f3.5-6.3 II VC PZD'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. Canon 20mm f2.8 USM weighs 405g, which means it is 45g (9%) lighter than the Tamron 18-270mm f3.5-6.3 II VC PZD which has a weight of 450g.
Filter Threads
The Canon 20mm f2.8 USM has a filter size of 72mm whereas the Tamron 18-270mm f3.5-6.3 II VC PZD has a 62mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 72mm filters that we recommend you to consider for the Canon 20mm f2.8 USM:
And these are the 62mm filters that we recommend for the Tamron 18-270mm f3.5-6.3 II VC PZD:
Lens Mounts
Both the Canon 20mm f2.8 USM and the Tamron 18-270mm f3.5-6.3 II VC PZD 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 18-270mm f3.5-6.3 II VC PZD 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 18-270mm f3.5-6.3 II VC PZD has a focal range of 18-270mm and 15.0X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 28.8-432mm when used on a APS-C / DX format camera.