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Mastering Canon EOS Flash Photography, 2nd Edition by NK Guy

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Appendix E: Sequence of Operation

E-TTL (film and digital) sequence of operation

The usual E-TTL sequence of operation, not counting the optional flash exposure lock (FE lock) feature or wireless operation, is as follows:

  • When the shutter release is pressed halfway, the current ambient light levels are metered by the camera as usual. Shutter speed and aperture are set by the camera or user depending on the current mode: PIC (icon) modes or P, Av, Tv, or M.
  • When the shutter release is pressed all the way, the flash unit immediately fires a low-power preflash from the main flash tube (i.e., white light).
  • The reflected light from this preflash is analyzed by the same evaluative metering system that the camera uses for metering ambient light levels. ...

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