I was also confused by the manual at first and thought it meant the flash tries to also expose the background . Some people think it is describing ''slow-synch'' type flash - they haven't explained it very well .I read this advice being given all the time, but don't understand what TTL/BL is all about.
According to the sb800 manual, when in TTL/BL mode, "the flash output level is automatically adjusted for a well balanced exposure of the main subject and background". But the flash doesn't illuminate the background, so how can it effect its exposure? It doesn't appear to change the shutter speed, iso, or aperture, which I'd think would be the only ways to effect the exposure of the background.
When I'm shooting flash outdoors, I take control of the background exposure by selecting the shutter speed, aperture, and iso, so I thought the only job left for the flash would be to expose the subject properly, which it TTL.
Would you mind explaining how TTL/BL works?
Basically from what I understand now is that it takes into account what the camera meter sees as well as the flash meter and doesn't think it is the only light source like TTL seems to do . I tried a few tests with TTL compared to TTL/BL outdoors and the TTL/BL always seemed to fire about 1 stop weaker .
Indoors was a different story and gave some interesting results so I stay with TTL indoors . I still end up setting TTL/BL to -1 outdoors as a personal preference .