#2. "RE: SB600, Gary Fong Lightsphere, vertical" In response to In response to 0
That is strange!
I assume you are in TTL flash mode?
Are you refurring to the light sensor window on the side of the SB-600?
That sensor is used to detect commands from the Master when the SB-600 is in Remote mode and should not affect the flash exposure at all when the SB-600 is in the cameras hotshoe.
Check the Ready light on the back of the Speedlight or in the viewfinder (lightning bolt symbol on the right side) after you capture an image. If it flashes, the SB-600 has fired at full power. Now check the image. If the image is under exposed, you will have to open the aperture and / or increase the ISO to get a correct exposure. Good Luck and Enjoy your Nikons!