Chapter 8. Working with Flash and Studio Lights
With the EOS 7D, the flash becomes not only a handy built-in accessory, but it serves for the first time as a wireless E-TTL (Evaluative Through the Lens) flash control. With this, the 7D opens the door to creative multiple and wireless flash photography.
You can use the 7D's Integrated Speedlite Trans-mitter as a master control unit to fire up to three groups of wireless slave units with or without having the built-in flash fire. The built-in flash can also be used as a separate flash group with reduced power output.
From the camera menu, you can control flash ratios or manually control the output of each group, and control the settings and custom functions for accessory Speedlites including the 580EX II, 430EX II, and the 270X from the 7D's Flash Control menu.
Flash Technology and the 7D
Canon flash units, whether it's the built-in or an accessory EX-series Speedlite, use E-TTL II technology. To make a flash exposure, the flash unit receives information from the camera including the focal length of the lens, distance from the subject, and exposure settings including aperture.
The camera gathers this information from a preflash beam that's fired after the Shutter button is fully pressed Information from this preflash is combined with data from the Evaluative metering system and the lens to analyze and compare existing light exposure values ...
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