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Part 2: Technical Information
With the conventional TIL Auto Flash Exposure System, the flash is cutoff with a signal
when the correct flash exposure level is attained. Excess charge remains after the flash
is cutoff. In the case of the E-ITL Auto Flash Exposure System, the flash output setting
obtained beforehand is used so the charge is equal to the required exposure. Therefore,
correct flash exposure is obtained by firing at "full power" without the need to stop the
flash with charge remaining in the capacitor. Flash control is therefore improved.
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SW1 On
:
SW2 On
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I
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1-a.
2-c.
Main flash
1-b.
Preflash
2-b.
Flash control signal
Fig. 2-16 E-TTL Autoflash System
2) Photography with E-TTL Auto Flash Exposure System
Flash exposure control for previous EOS cameras was designed with two objectives in
mind: to supply light in low-light situations and to provide fill-in flash for backlit sub-
jects. In other words, the flash was basically designed for use when ambient light alone
was insufficient. Therefore, when flash was used in an AE mode which set the correct
exposure for frontal light conditions, overexposure was likely. However, many photogra-
phers like to use medium-strength flash with frontal lighting.
The algorithm used by the E-ITL Auto Flash Exposure System is a major feature. It
allows the flash to adapt to various lighting conditions with the 6-zone metering sensor
which meters ambient light and the preflash. Mixed light readings are used to obtain a
natural balance between ambient light and flash.
2-19
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