Why doesn’t camera exposure compensation change hot-shoe flash exposure on a Canon XTi?
Asked 1/12/2011
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I’m using a Canon XTi with a Canon 550EX in E-TTL. When I changed the camera’s normal exposure compensation, it didn’t seem to affect the flash exposure. I found that I had to use flash exposure compensation instead.
If the flash output is being controlled by the camera through TTL/E-TTL, why doesn’t increasing exposure compensation on the camera also increase the flash output?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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On an intuitive level, you might think that exposure is exposure, and even though there are two different metering systems involved, they should talk to one another. That would be perfect if exposure was always either ambient or flash, but it stops short when you consider that flash is just as often used to augment ambient light, either to fill in shadows or to light a foreground subject in sharp relief against a darker background. Having the two systems work independently is a lot more useful in a lot more situations -- you may want to bracket the flash while leaving the ambient steady, or bracket the ambient with a constant flash, and so on. It does mean remembering that they're separate systems, but it really is better that way.
Originally by user2719. Source · Licensed CC BY-SA 4.0
user2719
15y ago
0
Generated from our catalog & community — verify before relying on it.
Because the camera treats ambient exposure and flash exposure as separate systems.
On the XTi, normal exposure compensation adjusts the camera’s metering for available light only. With a TTL/E-TTL flash like the 550EX, flash power is controlled separately using Flash Exposure Compensation (FEC).
That separation is intentional and useful. Flash is often mixed with ambient light, not just used as the only light source. You might want to brighten or darken the background with normal exposure compensation while keeping flash on the subject the same, or adjust flash output without changing the ambient exposure. Separate controls make that possible.
So in practice:
- Exposure compensation = ambient light metering
- Flash exposure compensation = TTL flash output
If you want the 550EX to put out more or less light in E-TTL, use FEC, either on the flash or in the camera if supported.
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