16-20?
What? Is that a misprint?
I bet they meant to type 16-28 and the 2 little circles of the 8 became a single big one for a zero...
16-20 makes no sense.
It combines the complexity and design compromises of a zoom lens without its single biggest benefit: zooming.
Horizontal view for FF is 96.7 degrees at 16mm and 84 degrees at 20mm.
You can achieve that by taking half a step forward.

(24Lii, sigma 50A, 135L), and for the other ends of the spectrum, sigmaEX 14mm2.8 and sigmaEX 100-300F4.
