I've got a Cosina 20mm wideangle m/f and it's pretty good. Cosina currently make lenses for Voigtlander I believe and also made the highly rated Vivitar Series 1 back in the day. Not to be sneered at.
Afaik, Cosina is primarily a third party manufacturer, used by other 'real-name' brands to outsource (some of) their own manufacturing. So Cosina produces stuff for other brands under those others' brand names. Like Vivitar and Nikon.
In the seventies and eighties Cosina marketed a few SLR's, and a limited range of lenses, under it's own name: Cosina. All those were also marketed by Cosina under the brand name 'Petri'. And who knows what other obscure 'brand names'.
Seems they're still with us and still going strong.
If you buy a new lens for your A-brand camera with that A-brand on it (and price to match), there's a better than even chance it wasn't manufactured in A-brand's own factories! But under license, contract.
By Cosina and their brethren.