You asked
Sigma 19mm f 2 8 dn art vs sony e 20mm f 2 8?
The Sigma 19mm f/2.8 DN Art and the Sony E 20mm f/2.8 are close cousins. Both are small, autofocus, unstabilised f/2.8 primes for APS-C, and both give about the same depth of field on a full-frame equivalent (roughly f/4.2).
The Sigma frames a little wider, about 28mm full-frame equivalent against the Sony's 30mm. It also mounts on Micro Four Thirds as well as Sony E, so it's the better pick if you might switch systems. The Sony is the native choice if you shoot a Sony E camera and want a lens made by the body's maker.
For most Sony APS-C owners, I'd go with the Sigma 19mm f/2.8 DN Art for the slightly wider view and the mount flexibility. If you want the Sony-badged option, the Sony E 20mm f/2.8 is just as sensible.
Written by Claude from this site's pages.