You asked
What is the sigma 50 120mm f 2 8 dc os contemporary good for?
The Sigma 50-120mm f/2.8 DC OS Contemporary is a compact, constant f/2.8 telephoto zoom made for APS-C cameras. On APS-C it frames like roughly 75-180mm on full frame, so it suits sports, wildlife, events, and portraits where you want to stand back and still get a tight shot with soft background blur. It's stabilised and has autofocus, which helps when you're shooting handheld at a distance.
It's a lens for the subject you can't walk up to, not for landscapes or street work. Mounts are Sony E, Fujifilm X and Canon RF, and it fits a long list of bodies, from the Sony a6000 and Fujifilm X-T30 II up to full-frame bodies such as the Sony a7 IV and Canon EOS R6 Mark II, where it works in APS-C crop mode at fewer megapixels.
For portraits, I'd still point you to the Portraits guide, which covers the 135mm and 70-200mm options for tight headshots. For a deeper look at how crop factor changes depth of field, see the Focal length converter.
Written by Claude from this site's pages.