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 AF 18-200mm f3.5-6.3 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. Tamron AF 18-200mm f3.5-6.3 XR II LD is the longer of the two lenses at 84mm. The Canon 20mm f2.8 USM with a length of 71mm, is 13mm shorter. On the other hand, the Canon 20mm f2.8 USM has a larger diameter of 78mm compared to the Tamron AF 18-200mm f3.5-6.3 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 AF 18-200mm f3.5-6.3 XR II LD weighs 398g, 1% (7g) lighter than the Canon 20mm f2.8 USM's weight of 405g.
Filter Threads
The Canon 20mm f2.8 USM has a filter size of 72mm whereas the Tamron AF 18-200mm f3.5-6.3 XR II LD 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 AF 18-200mm f3.5-6.3 XR II LD:
Lens Mounts
Both the Canon 20mm f2.8 USM and the Tamron AF 18-200mm f3.5-6.3 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.
Tamron AF 18-200mm f3.5-6.3 XR II LD is also available in Nikon F (DX) , Sony Alpha and Pentax KAF mounts.
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 AF 18-200mm f3.5-6.3 XR II LD has a focal range of 18-200mm and 11.1X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 28.8-320mm when used on a APS-C / DX format camera.