>I don't know why I am still having trouble wrapping my head >around the SB800. When the flash head is at 90 degrees >(shooting direct at the subject) in TTL, I can get perfect >exposure. Now, if I put the diffuser on the flash and the head >is still 90 degrees and TTL, does the flash automatically >compensate for light loss through the diffuser?
Yes! As long as there is sufficient reserve power, the exposure will be the same.
>Now, if I were >to bounce the flash at say 45 degrees, does the flash >compensate for any bounce for the head being at 45 degrees?
Yes! TTL (Through The Lens) flash metering detects the monitor preflash and adjusts the flash power to provide a correct exposure. As long as the Speedlight has sufficient power.
>Another question: If you have the camera set at say 5.6 1/60 >sec. 400 iso, and you bounce the flash off a white ceiling, I >get an underexposure. Now, if I open the aperture to say 4.0, >will the flash then try and compensate for the larger opening >by automatically powering down the flash because of the TTL or >will the flash power remain the same only giving me better >exposure because of the larger aperture?
If you were getting underexposure at f/5.6, the Speedlight was almost certainly already fireing at full power. If the red ready light (on the back of the SB-800) and the Ready Light (Lightning Bolt symbol in the viewfinder) is flashing the after the flash fires, the Speedlight has fired at Full Power. When you set the aperture to f/4 the one less stop of flash power is required for a correct exposure. If the first image was underexposed by one stop or less (assuming the Speedlight fired at Full Power), the second image will be exposed correctly since the flash will have enough power.
>I guess you could say >that I am still having trouble with under exposure with my >flash when I bounce or use a diffuser and don't know how to >compensate for the light loss.
After the flash fires, check the Ready Light and see if it is flashing. If it is, and the image is underexposed, open the aperture and or adjust the position of the flash head and or Diffuser and try again. Good Luck and Enjoy your Nikons!