On the main page I've discussed how TTL/BL works differently to TTL by also using the focus points to meter from . It is not foolproof though , it has it's weaknesses . The main one being bounce flash in the presence of white subjects - or white in the frame .
With a fairly neutral subject it is similar to TTL .
Even when we move further back things look ok .....
Then we introduce something white in line with one of the focus points .....
and it almsot behaves like spot metering looking for the brightest part of the frame and under exposes . +1 flash compensation recovers this loss of exposure but it is a nuisance to have to do this . Actually it should probably be +1.3 stops to get the white's ''white'' .
There is good news though , it is consistent in its underexposure unlike TTL metering . We can attain TTL metering by having the camera in manual mode and changing to spot metering , then the SB400 reverts to TTL metering if that is what you are familiar with and prefer to use .
I would say you need to make a decision between using the SB400 in manual and spot metering to get TTL flash along with FV-lock OR use it in normal matrix or centre weighted metering and set one compensation value for any picture with at least one focus point on a white subject .
To show you what I mean the next three pictures are what TTL flash does with a subject that is not always central , and with varying colour backgrounds- or subjects for that matter .
The right side of the histogram is all over the place ....
Now compare the erratic metering of the above pictures to the consistency of TTL/BL+1 .
See how all the histograms in TTL/BL+1 are in the same line ! As long as at least one focus point was on that white background the exposure stayed in the same place !
The D90 only allows +1 flash compensation but I can get more by dialing it in to normal EV compensation - it is cumulative , if I dial in +1 flash compensation and +0.7 EV compensation I will actually have +1.7 flash compensation .
I find this to be the biggest weakness with TTL/BL . It means either resorting to spot metering for normal TTL flash or watching what is in the frame carefully .
The consolation is that if you know there will be a light coloured object somewhere in the frame all the time that one exposure compensation will fix it with consistent results as long as at least one focus point is on that light coloured object .
No flash mode is perfect which is why it is good to learn how they behave - especially if you have an SB400 which only does TTL/BL unless you resort to spot metering !