Translation:Twenty years ago the whole world knew this camera.
Very odd sounding in English. The proper phrase would be "20 years ago the whole world knew about this camera" The way Duo constructs it leads me to believe the the whole world knew about that one individual camera. Like it was owned by Elvis and was auctioned for 50 million dollars... and everyone knew about that ONE camera.
It can mean "everybody" without any restrictions. "Весь мир знает этот фотоаппарат" means that the camera is known in France, Japan and Chile etc. "Весь мир знает, что это ты украл деньги" rather means that everybody knows that. Not exactly everyone in US and China, but the fact is not a secret and is widely known.
"Знать" is always followed by the accusative. But for some nouns (masculine animate for example) the accusative and the genitive forms happen co coincide.
Also in the negative: "doesn't know", the genitive case is sometimes acceptable, but it's rare and it's never required, only serves as a possible alternative to the accusative. It's an outdated form that simply hasn't died out completely yet.