TTL/BL increase in exposure with diffuser dome .

The manual suggests using the diffuser dome at 60 degrees for bounce flash .I have found that generally TTL/BL and wireless CLS under-exposes with bounce flash with white objects .
I noticed recently in some tests using the diffuser dome [ which I seldom use ] that the exposure seems to increase when it is used .
With plain TTL flash there seems to be a slight increase in exposure when the diffuser dome is on , possibly something to do with reflectivity and the way the metering pattern of TTL works .

With TTL/BL though the change is a lot more pronounced .

There's a slight increase in exposure when the wide angle diffuser is pulled out and the flash head pulls back to wide angle inside .

Then there is another slight increase when the diffuser dome is fitted .

Now when you fit the diffuser dome there is a small button under the flash head that gets pressed in . Watch what happens when I simply press that button ...... the flash head zooms back and BAM !

To get the same increase in exposure with direct flash I had to dial in an extra two stops of compensation .
So there is something in the TTL/BL program that responds to that button , besides the fact that the flash head zooms back to its widest setting it also seems to increase its output excessively disregarding the pre-flash reading .
Another possible concept is that when it expects a wide flash output and more diffused light perhaps the program that detects reflections and disregards them from the pre-flash reading starts to allow for wider areas of reflection resulting in over-exposure as it starts reading more from the darker areas . [ these are all just theories for now ] .

Now in the past I have noted that TTL/BL under-exposes quite badly with white objects and bounce flash but this increase in output seems to counter-act that under-exposure if you remember to either use the diffuser dome or the wide angle diffuser - both of which increase the output .
The manual for the SB800 [ page 101 ] states that best results are achieved when the flash head is at 60 degrees with the diffuser dome on [ depending on the situation of course ] , perhaps they have calculated their over and under exposure and the amount of light the dome throws forward at this setting  ?
Either way I tried the same subject with bounce flash ......
And of course we have the usual over-reaction to whites that TTL/BL displays with bounce flash .

Then we fit the diffuser dome .... much better !

And once again either pulling out the wide diffuser or simply pressing the button under the flash head that the diffuser dome switches still gives us a favourable increase in exposure ...

So my thoughts for now are that if you use bounce flash then either have the wide diffuser out , or the diffuser dome , or be prepared to dial in positive flash exposure compensation with 'normal ' bounce flash  .

I was also requested on a forum to post a picture of the room .....

Update : some tests with bounce flash and distance info ...

When you use plain bounce flash with no modifiers the TTL/BL program "almost" ignores the distance info - we'll call it a 95% pre-flash reading and a 5% distance reading calculation just to make it easier to explain .

The pre-flash reading identifies the white ball as the subject and responds by aiming for a neutral gray output .

Changing the focus and obviously the distance information from the lens has a very small effect on the exposure .

As soon as the diffuser dome is added or the wide diffuser is pulled out [ Just in case the program assumes that the bounce card will be used I left it extended ]  the distance info affects the output to a larger degree and this shows in the change in focus [ and the resulting change in distance info ] .

It's starting to look more like a 50/50 equation now with regard to input from the pre-flash reading and the distance info .

The histograms are almost exactly the same with the wide diffuser and bounce card .

This explains the 'superior' output with bounce flash when the diffuser dome is used but doesn't really explain why the output increases with direct flash .

I also did some tests with the SU800 and one flash in wireless mode .
There was no change with focus distance showing that distance info doesn't seem to be any part of the equation with wireless flash .
There does still seem to be that slightly better look and exposure with the diffuser dome though .....