When are you allowed to switch on rear fog lamps?

You answered correctly. Rear fog lamps are only allowed when fog reduces visibility to less than 50 meters. This is a strict rule in Germany to prevent dazzling other drivers. In situations like rain or visibility greater than 50 meters, you must not use rear fog lamps. Always use them only in heavy fog when you can see less than 50 meters ahead, for example, when you cannot clearly see the marker posts along the road. This ensures safety for everyone and avoids unnecessary glare.