被动语态一定要有介词吗

其实没有必要这么较真,被动语态表示被动,从理论上来说是“被..."肯定要有介词,但是被的对象不明确的话就没有必要写出来.His plays are known all over the world.=His plays are known by the world.
His plays are turned into wonderful films.这句话”被...”省略掉了,完整应该是：His plays are turned into wonderful film by sb.