There has been much confusion on the subject of TTL/BL . I don't know the full history but this is my 'theory' based on what I do know and the tests I have done .
Nikon have changed how TTL/BL works without explaining much to us .
If anyone tells you that you need a bright background for TTL/BL to work ask them why Nikon made the SB400 to only work in TTL/BL mode if they knew beginners will be using it in all conditions !?! Ask them why the built in speedlight and Nikon's wireless CLS meters in TTL/BL mode ?!!
The concept is not so hard to accept is it ?
Early cameras : TTL/"BackLit" , camera sends info about ambient on subject/[centre of frame] and background/[ the rest of the pattern , matrix or Centre Weighted ] , flash fires at the correct power to make the subject equal to the background light . - needs FV-lock if your subject is not central , not so with the "new" TTL/BL because now it uses all the focus points to meter .
Newer cameras : TTL/"BaLanced fill flash " - it was too much trouble to change the name on the flash heads so they threw the word 'balanced' into the equation . Nikon decide to use this superior technology with all situations along with the metering off the focal points . Now you can spot meter off any focus point , so they are metering off all the focus points for TTL/BL ! Todays cameras seem to meter strongly toward the focus points so it seems to be part of their 'evolution' .
With the focus point metering now available why not just ignore the background and use it to concentrate on superior exposure on the subject in any situation !
So they change what the camera tells the flash , the flash still thinks it's balancing the 'subject' with the 'background' but let's say for example they send 'fake' information about the background telling the flash it is perfectly exposed and then send the 'subject' info as the focus point readings- the flash tries to 'balance' the 'subject' with what it thinks is a perfectly exposed ' background ' and basically now it works in any situation - the flash tries to get perfect exposure of the subject regardless of the background !
Try it for yourself if you don't believe me . Use an off-centre subject to really test it because a totally central subject will give almost exactly the same results as TTL flash . Try a black background indoors or go outside at night [ because the 'experts' say it needs a bright background to work or it will fire at its weakest and underexpose ] . take a test shot in TTL and TTL/BL and you will see that TTL/BL flash does work in low ambient - better than TTL !
If you introduce a white object under one of the focus points it will underexpose but it's consistent so one exposure compensation should do for any further pictures with that white object under a focus point [ It's spot metering off the focus points ! ] . plain TTL will give varying results .
So : TTL/BL does NOT need a bright background to work and it does NOT need the subject to be central ..... anymore .
I just did some quick tests to confirm the fact that when using direct flash indoors [ which we probably never will do ] , no compensation is needed because of the superior distance info . That is , with the built in speedlight , and the SB400/600/800 when facing forward - the flash is constant .For a while I wondered then , why the output varies with direct flash outdoors in sunlight with different reflectivity if it is constant indoors ?
I think the reason it is accurate indoors with direct flash is that it must use the focus points to determine the ambient as well . It detects no ambient and adds the flash needed for correct eposure at that distance . I am beginning to doubt that the metering pattern is used at all which may be how they 'bypassed' the original program that required ''background'' lighting to balance with .The old program used the brightest part of the outside of the metering pattern as the 'background' to balance with , calculated how much ambient was on the centre of the frame , and added the correct amount of flash to equalize them . The new program , perhaps , uses the brightest focus point that detects the pre-flash as the subject , and adds what it deems to be necessary to get an average grey exposure . It still puts out the right amount of flash it would need with direct flash , less what it deems to be the ambient on the subject - which does get fooled by the reflectivity of the subject . Even though it could give 'perfect' exposure when there is no ambient due to the distance info it gets from the lens - when it detects ambient it is also at the mercy of the focus points metering off varying subject colours .By using the brightest focus point for its metering it would [ theoretically ] ensure that there are no blown highlights as long as the object is under a focus point . So perhaps that is what Nikon changed in the cameras to fool the old "back lit" system . A simple theory is that instead of sending the outer metering reading to the flash , they send a fake reading that says it is perfectly exposed there , and then instead of sending the centre of the pattern as the subject info they send the info from " the brightest focus point detecting the pre-flash " as the subject reading .Either way , that's how it appears to work .