TTL Flash Compensation in Nikon CLS

Started Jun 13, 2006 | Discussions thread
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Jim Kaye Senior Member • Posts: 2,752
TTL Flash Compensation in Nikon CLS

I was doing some informal portraits recently and was having trouble getting the fill ratio I wanted when using two SB-800’s wirelessly in TTL mode, which made me realize I wasn’t sure I understood exactly how TTL compensation works in the CLS. So tonight I set up a simple test with two speedlights, one as master (on-camera) and the other as remote (45 degrees to one side) and shot a Kodak grey card, which filled the entire frame of my shot. This was indoors in low ambient light where the test object would have been more than 3 stops underexposed without flash illuminating it. I started simply and worked up to something slightly more interesting, so please bear with me for a few paragraphs...

When I set the remote to “TTL 0.0” and the master to “—“ the exposure was perfect (narrow histogram peak just about at the midline of the display). The same was true if I switched the settings so the master was “TTL 0.0” and the remote was “—“. Conclusion: either master or remote can produce the correct exposure as a single flash when no TTL compensation is set. No surprise there.

Next I set both flashes to “TTL 0.0” and the exposure was again perfect. Conclusion: the camera can reduce the output of two flashes so that they complement one another perfectly to produce a correct exposure. This is of course just what one wants, and again is no surprise. But think about what it means. The camera alters the output that “TTL 0.0” represents when there is a second flash contributing to the exposure. So “TTL 0.0” doesn’t represent some fixed output (even in this simple setup). It represents less output when there is a second flash in the mix.

Next, to simulate a key light from the side and on-camera flash as fill, I set the master to “TTL –2.0” and the remote to “TTL 0.0.” This shot was underexposed by a stop or so, which surprised me -- I was expecting a "correct" exposure again (since one of the flashes was set to plain old uncompensated TTL). I got the same underexposure if the master was set to “TTL 0.0” and the remote was set to “TTL –2.0.” Conclusion: When TTL flash compensation differs between two flashes in the CLS, the camera seems to try to make the overall exposure an average of the two settings. I confirmed this by setting the master to “TTL –1.0” and the remote to “TTL +1.0.” The histogram peak was now right at the midline. The same happened when I swapped the setting on the master and the remote. So in theory this is how I need to set the system up to get a correct overall exposure with a two-stop difference between the key and the fill (rather than key "TTL 0.0" and fill "TTL -2.0").

The compensation to the TTL setting are not “absolute” – that is, they do not represent fixed deviations from the output corresponding to “TTL 0.0” for each flash (which is of course not fixed either). Rather, TTL compensations appear to be relative to the average of all the flashes that are set at various TTL levels (whether that average corresponds to a “correct” exposure or not). Of course this was a ridiculously simple setup and the camera could “see” the output of both flashes very clearly – both were aimed right at the grey card. With a 3-dimensional subject, the camera might not “see” the full output of a flash positioned to light the subject from the side, for example. And if exposure compensation were also set on the camera, that would modify the “average” set point for the TTL flashes (as well as the background exposure if there were enough ambient light), making things even more complicated.

Nikon really doesn’t provide enough documentation of how CLS works, IMHO, but that’s another issue...
--
Jim Kaye

'I believe that the electronic image will be the next major advance. Such systems will have their own inherent and inescapable structural characteristics, and the artist and functional practitioner will again strive to comprehend and control them.' -- Ansel Adams, 1981

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