Ariel S , Nov 09, 2011; 11:41 p.m.
A dim little tungsten light is hardly adding anything. When you'll see a difference with TTL/BL is when you're using the camera with plenty of ambient light. If you're using a dim tungsten light, even if your shutter speed is 1/60, your flash is overpowering, so you won't see a difference. If you're just bouncing the flash if you're in the shade, to provide another light source for creativity, or you're using fill flash in daylight, you'll notice a difference. TTL/BL will take into account the exposure that your camera would make if you didn't have a flash.