#3. "RE: Need advice: 35 f1.8G or ( 50 f1.8D + flash Youngnuo )" In response to In response to 2
To be more clear, I would recommend getting a Speedlight for shooting events and proms in low light situations. Nikon DSLR's like your D90 use i-TTL flash control that is not compatible with Film TTL flash control or the earlier D-TTL flash control. Third party or aftermarket Flash umits or Speedlights may not support i-TTL flash control and others support it to varing degrees of sucess. The Nikon SB-700 and discontinued SB-600 fully support i-TTL flash control. The latest Metz flashes are also fully compatible. There are a few Sigma flash units as well as Nissen flash units that support I-TTL to some degree depending on the model. You can ask questions about Flash and Speedlight compatibility in the Speedlight forum. Good Luck and Enjoy your Nikons!