The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 20mm F1.8 EX DG RF and the Sigma 18-50mm F2.8-4.5 DC OS HSM 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. Sigma 20mm F1.8 EX DG RF is the longer of the two lenses at 90mm. The Sigma 18-50mm F2.8-4.5 DC OS HSM with a length of 89mm, is 1mm shorter. Besides being longer, the Sigma 20mm F1.8 EX DG RF also has a larger diameter of 89mm compared to the Sigma 18-50mm F2.8-4.5 DC OS HSM'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. Sigma 18-50mm F2.8-4.5 DC OS HSM weighs 395g, 24% (125g) lighter than the Sigma 20mm F1.8 EX DG RF's weight of 520g.
Filter Threads
The Sigma 20mm F1.8 EX DG RF has a filter size of 82mm whereas the Sigma 18-50mm F2.8-4.5 DC OS HSM has a 67mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 82mm filters that we recommend you to consider for the Sigma 20mm F1.8 EX DG RF:
And these are the 67mm filters that we recommend for the Sigma 18-50mm F2.8-4.5 DC OS HSM:
Lens Mounts
The Sigma 20mm F1.8 EX DG RF has the Nikon F (DX) lens mount whereas the Sigma 18-50mm F2.8-4.5 DC OS HSM has the Canon EF lens mount. Some of the latest released cameras that are compatible with these mounts are Nikon D6, Nikon D780 and Nikon D3500 for the Nikon F (DX) Mount and Canon T8i, Canon 90D and Canon SL3 for the Canon EF Mount.
Sigma 20mm F1.8 EX DG RF is also available in the Canon EF , Pentax KAF, Sony Alpha and Sigma SA mounts.
Sigma 18-50mm F2.8-4.5 DC OS HSM is also available in Pentax KAF3 , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
Sigma 20mm F1.8 EX DG RF 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.
On the other hand, the Sigma 18-50mm F2.8-4.5 DC OS HSM has a focal range of 18-50mm and 2.8X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 28.8-80mm when used on a APS-C / DX format camera.
