There are three choices for this menu choice. (The Flash mode entry is grayed out and cannot be selected, as the built-in flash mode is fixed at the E-TTL II setting.)
■ Shutter sync. You can choose 1st curtain sync, which fires the flash as soon as the shutter is completely open (this is the default mode). Alternatively, you can select 2nd curtain sync, which fires a preflash used to measure exposure as soon as the shutter release is pressed down all the way, but before the shutter opens, and then triggers a second flash at the end of the exposure, just before the shutter starts to close. This action allows photographing a blurred trail of light of moving objects with sharp flash exposures at the beginning and the end of the exposure, as I described earlier in this chapter.
■ Flash exp. comp. This is an alternate method of setting flash exposure compensation. Select with the Set button, then dial in the amount of flash EV compensation you want using the left/right cross keys. The EV that was in force prior to your adjustment is shown with a blue indicator. Press Set again to confirm your change, and then press the Menu button to exit.
■ E-TTL II. You can choose evaluative or average metering modes for the electronic flash exposure meter. Evaluative looks at selected areas in the scene to calculate exposure, while average calculates flash exposure by reading the entire scene.
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