ETTL & flash exposure compensation

Started Feb 11, 2013 | Discussions thread
beagle1 Veteran Member • Posts: 7,170
Re: ETTL & flash exposure compensation

John Deerfield wrote:

I use my camera in full manual mode. My flash settings for the 600ex are as follows: ETTL, manual zoom, and the output is 1/1.

Can someone please explain the difference in turning down the output in the M mode on the 600ex compared to turning down the output in ETTL mode?

If the flash is in the manual mode, there is no pre-flash. The flash is simply firing at whatever power you have set it for. You could have it set for 1/1 (full power); 1/2 power, 1/4 power and so on. If the flash is set to TTL, then the camera is deciding at what power the flash will fire. To make this determination, there is a pre-flash that the camera reads in order to gauge how much light/power is needed from the flash. There isn't any flash exposure compensation when the flash is in the manual mode. There isn't anything to compensate for as you dialed in the power. In TTL, the flash metering system works just like the camera's metering system in so much as it wants to make the world gray. As example, you take a picture of a white wall using the flash as your main light source. In TTL, you will get a gray wall since that is how metering works. Now, you can dial in positive flash compensation letting the camera know that we need more light than what it thinks. In turn, we get a brighter (whiter) wall. On the other hand, if the flash was in manual mode and you did not get enough light output from the flash, you would need to increase the power yourself. For more about on camera flash read this:

good explanation and link

I generally leave the flash in ETTL mode and the camera in manual mode

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