The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Samyang 20mm F1.8 and the Tamron 18-270mm f3.5-6.3 II VC PZD 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. Tamron 18-270mm f3.5-6.3 II VC PZD weighs 450g, 9% (47g) lighter than the Samyang 20mm F1.8's weight of 497g.
Filter Threads
The Samyang 20mm F1.8 has a filter size of 77mm 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 77mm filters that we recommend you to consider for the Samyang 20mm F1.8:
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 Samyang 20mm F1.8 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.
Samyang 20mm F1.8 is also available in Canon EF-M , Fujifilm X, Micro Four Thirds, Nikon F (FX), Pentax KAF, Samsung NX, Sony Alpha and Sony E mounts.
Tamron 18-270mm f3.5-6.3 II VC PZD is also available in Nikon F (DX) mount.
Focal Range
Samyang 20mm F1.8 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.