You asked

Is the sigma 20 200mm f 3 5 6 3 dg contemporary good for portraits?

Here is what the site says about that.

From Sigma 20-200mm f/3.5-6.3 DG Contemporary: Sigma 20-200mm f/3.5-6.3 DG Contemporary: A zoom lens by Sigma for Full frame cameras, 20-200mm f/3.5-6.3. A 10x full-frame travel zoom that starts at 20mm. Specs - Maker: Sigma - Focal length: 20-200mm (zoom) - Maximum aperture: f/3.5-6.3 (it gets darker as you zoom in) - Made for: Full frame sensors - Mounts: Sony E, L-Mount - Stabilised: no - Autofocus: yes What it frames like - On full frame (Sony E, L-Mount): as marked, ...

From Portraits: Portraits: For portraits, the lens matters far more than the camera. A cheap body with a good portrait lens beats an expensive body with a kit zoom every time.

Quoted from this site's pages.

From these pages