Saturday, April 3, 2010

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 .....

8 comments: said...

I wonder if the wide area AF assist function is involved ? When engaging either the dome or the wide flash adapter the zoom defaults to 14-17mm and if the AF assist is engaged might explain the better coverage.

Desmond Downs said...

The thing is that with bounce flash the whole scene is evenly lit anyway regardless of whether the dome is on or not . You would think the metering would be the same .
The fact that pushing the button only , compared to fitting the diffuser , really over-exposes makes it seem like it is programmed to compensate for wasted energy -but why can't it just meter the light ???

Russ MacDonald said...

Because it uses both distance and metering when in TTL-BL mode.

It calculates a portion of the flash power based on the metered preflash measurement and the other portion is based on the focus distance (assuming a D lens).

There is a fairly complex equation that is calculated by the flash computer that analyzes the ambient light data from the matrix or center-weighted metering, the distance to the subject, and whether the diffuser dome is on or not (button is pressed or not).


Desmond Downs said...

I understand the part about distance info and the pre-flash reading - but why would it increase output so much for the diffuser dome compared to without ? Especially with direct flash where it is already using distance info !

Russ MacDonald said...

Yes, that's what I meant. It assumes a loss for the diffuser.

Also, it uses distance information even when bouncing. It's just that the distance doesn't have as much effect when bouncing.

The BL equations that determine the flash power are very complex and involve just about every parameter there is.


Desmond Downs said...

Thanks for the input - I've added a few more tests . When there is no form of diffusion used the distance info has very little effect - but when the wide diffuser or the diffuser dome is used it increases usage of the distance info in the equation .

Russ MacDonald said...

I was reading your comments again, and I was reminded of one more thing.

In TTL-BL mode, When the preflash return is strong much more emphasis is placed on the preflash and much less is placed on the distance.

In TTL-BL mode, when the preflash return is weak, much more emphasis is placed on the distance.

This may be part of the differences you are seeing between using the diffuser and not using the diffuser. The diffuser can cause a big difference in the preflash return energy.


Desmond Downs said...

That is a very interesting piece of information Russ , like you mentioned a while ago about the distance info being there in case the pre-flash reading fails in bright sunlight [?] , thanks for the input !