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CLS i-TTL Power Settings Independent of Group

bclaff

Vancouver (WA USA not BC Canad, US
9691 posts

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bclaff Awarded for multiple contributions for the Resources Registered since 26th Oct 2004
Fri 12-Sep-08 12:39 AM

Hal's (HBB) recent post reminded me to perform this test.

I shot 4 images of the same target with two SB-800s in TTL mode using an SU-800.
One SB-800 was in Group A. The other was in Group B.
Here are the resulting images, identically processed.

Click on image to view larger version


Upper left is using Group A and Group B
Upper right is using Group A
Lower left is using Group B
Lower right is using Group A and Group B but Group B was not pointed at the subject

If it isn't clear from the images, here are the histograms.

Click on image to view larger version


Clearly when Group B did not point at the subject; Group A was not increased in power to compensate.

I had hoped that the Nikon system was smarter than this.
That it would do all the pre-flashes and then compute all the powers.
But this test appears to confirm that power levels are set on each Group based on it's pre-flash (for i-TTL).

The practical implication would seem to be that is that it's unwise to illuminate the same portion of a subject with flashes from different Groups as their interaction is not taken into account.
(Typically, when I use more than one Group, I'm using one for "foreground" and another for "background"; and so I have never run afoul of this issue myself.)


Bill


Attachment#1 (jpg file)
Attachment#2 (jpg file)

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