CLS Exposure Question

Started Jun 21, 2007 | Discussions thread
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Jim Kaye Senior Member • Posts: 2,751
Re: CLS Exposure Question

This is a complicated question. Presumably you are using an auto-exposure mode on the camera (A, S, or P) and either TTL or TTL-BL? And are you using one flash or are you using one to trigger the other remotely (as well as perhaps to light the scene some itself). Technically a single flash is also part of the "Creative Lighting System" although you may be using the term CLS to mean wireless remote flash.

If you are using TTL (rather than TTL-BL) and a single flash attached directly (or through a cord) to your hotshoe is providing most of the light on the subject (such as shooting a person outdoors at night with a low enough ISO, small enough aperture, and fast enough shutter speed that the ambient light doesn't contribute much to the exposure), then changing exposure compensation and changing flash compensation should have pretty much the same effect. (They aren't exactly the same if you measure carefully under a wide enough range, but they have close enough to the same effect to consider them the same for practical purposes.)

If there is more ambient light, the ISO is higher, the aperture is wider, and/or the shutter speed is slower (say, normal indoor lighting) so that the ambient light contributes to the exposure, then the effect of exposure compensation and flash compensation for a single flash will differ. Exposure compensation will increase the expousre of the subject lit by flash as well as the exposure of the background. Flash compensation will increase only the exposure of the subject (assuming the background is far enough away not to be lit mostly by the flash). In a typical living room, the difference may not be so obvious because the flash may provide more light to the background than the ambient light source does.

If you're using TTL-BL or multiple wireless flash (which is always a form of TTL-BL), and the ambient light, ISO, aperture, and shutter speed are such that the ambient light contributes to the exposure, it is more complicated. The camera may adjust both exposure compensation and flash compensation to provide what its little eyes and brain think is a "pleasing" balance of flash and ambient light. Whatever you dial in for exposure compensation or flash compensation may be overridden or ignored, and you don't know what the camera is doing, ultimately.

This is one of the reasons why experts like Thom Hogan recommend for flash photography that you use manual camera exposure settings (to control how much ambient light contributes to the background exposure) and use TTL (rather than TTL-BL) with flash compensation (set either on the camera or the flash but preferably not both, because they are additive) to determine how much the flash illuminates your subject. This approach has the additional advantage that it allows use of all apertures on your lens (which shooting in P mode won't) and that it encourages you to think about different situations (flash providing the main light on subject, flash providing only fill light on subject, etc.) in the same way.

I'm sure Thom or others will jump in here if I've misstated or omitted anything.
--
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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