i have a small problem with sb900 flash or maybe with my d700 camera. i shoot interiors and set the camera to manual (e.g. 1/125 iso 400 f4), take more shots ok and after a while it starts to overexpose. the flash is set up on ttl bl. how can i solve that? where seems to be the problem?
TTL BL is Nikon's way of meaning 'Balanced Fill Lighting'. I believe what it means is that in 'BL' mode, the camera will attempt to 'balance' the exposure of the subject (i.e. a person in the photo) with the background. If there is no subject, the flash is probably metering its exposure on whatever the focus sensor (black square) of the camera is pointing at and focused on. Simply moving the camera around in a room will constantly yield different results as the flash system is attempting to meter on your 'subject' and 'balance' it with the background. If there is no real 'subject' vs. background then BL is not the mode to use.
I rarely use BL mode. I find it yields unpredictable results.
I think a better strategy might be (as mentioned in the other replies) to set your background exposure in manual, as you have done, leave the flash in TTL only, adjust the flash compensation on the flash as desired (e.g. -0.7 to -1.7 for things, people, etc.). That way you are 100% in control of the camera and flash exposures rather than letting the camera try and figure out what you want to do (BL mode.
If you are shooting a kitchen, set the camera to manual, pick the correct exposure settings without considering the flash, the turn on the flash, set to TTL mode, adjust the flash comp +/- as desired. Then you could theoretically move the camera around the kitchen or room and get the same results with every shot. That's not likely to happen in BL mode.
I have been shooting for some time with Nikons and SB800s, but I bought an SB900 at the same time I purchased the D700 and am having the same overexposure problems with the SB900. I ended up putting it in a drawer and went back to using the old SB800, as its TTL BL exposure is more predictable and much more reliable. Sorry but this is my take on the new SB900!