The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Tamron 18-270mm f3.5-6.3 II VC PZD and the Samyang 20mm F1.8 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, which means it is 47g (9%) lighter than the Samyang 20mm F1.8 which has a weight of 497g.
Filter Threads
The Tamron 18-270mm f3.5-6.3 II VC PZD has a filter size of 62mm whereas the Samyang 20mm F1.8 has a 77mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 62mm filters that we recommend you to consider for the Tamron 18-270mm f3.5-6.3 II VC PZD:
And these are the 77mm filters that we recommend for the Samyang 20mm F1.8:
Lens Mounts
The Tamron 18-270mm f3.5-6.3 II VC PZD has the Nikon F (DX) lens mount whereas the Samyang 20mm F1.8 has the Nikon F (FX) lens mount. Some of the latest released cameras that are compatible with these mounts are Nikon D3500, Nikon D7500 and Nikon D5600 for the Nikon F (DX) Mount and Nikon D6, Nikon D780 and Nikon D3500 for the Nikon F (FX) Mount.
Tamron 18-270mm f3.5-6.3 II VC PZD is also available in Canon EF mount.
Samyang 20mm F1.8 is also available in Canon EF , Canon EF-M, Fujifilm X, Micro Four Thirds, Pentax KAF, Samsung NX, Sony Alpha and Sony E mounts.
Focal Range
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
27-405mm when used on a APS-C / DX format camera.
On the other hand, the 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 1.5x crop, it provides a 35mm (FF) equivalent of 30mm.