You asked

Sigma 18 50mm f 2 8 dc dn contemporary vs sony e 16 55mm f 2 8 g?

Here is what the site says about that.

From Sigma 18-50mm f/2.8 DC DN Contemporary: Sigma 18-50mm f/2.8 DC DN Contemporary: A zoom lens by Sigma for APS-C cameras, 18-50mm f/2.8. A tiny constant f/2.8 standard zoom: the best upgrade from a kit lens on APS-C. Specs - Maker: Sigma - Focal length: 18-50mm (zoom) - Maximum aperture: f/2.8 - Made for: APS-C sensors - Mounts: Sony E, Fujifilm X, L-Mount, Canon RF - Stabilised: no - Autofocus: yes What it frames like - On APS-C (Sony E, Fujifilm X, L-Mount): ...

From Sony E 16-55mm f/2.8 G: Sony E 16-55mm f/2.8 G: A zoom lens by Sony for APS-C cameras, 16-55mm f/2.8. Specs - Maker: Sony - Focal length: 16-55mm (zoom) - Maximum aperture: f/2.8 - Made for: APS-C sensors - Mounts: Sony E - Stabilised: no - Autofocus: yes What it frames like - On APS-C (Sony E): like 24-82mm on full frame; f/2.8 gives about the depth of field of f/4.2 there. - On the larger-sensor bodies of Sony E: ...

Quoted from this site's pages.

From these pages