Canon EOS 20D Service Manual page 88

Technical Information
The basic idea for maintaining a consistent and stable exposure even after
the shot is recomposed remains the same as before. Also, the camera's
accurate decision to fire the built-in flash based on the orientation sensor
reading also comes from the same, previous idea.
(4) E-TTL II algorithm
E-TTL II autoflash control takes effect when the built-in flash or an EX-
series Speedlite is used. The EOS 20D also uses the EOS-1D Mark II's E-TTL
autoflash algorithm which is more advanced than the EOS 10D's E-TTL
autoflash algorithm.
AF point-linked evaluative metering control abolished:
In past EOS cameras, evaluative metering was based on the assumption
that the AF point covered the main subject. However, with the automatically-
selected AF point or the manually-selected center AF point, the spot where
you want the flash exposure reading sometimes was not the same as the
point of focus. Therefore, the desired flash exposure was not obtained. The
EOS 20D's evaluative flash metering control uses an algorithm that keeps the
flash exposure area separate from the AF point.
Subject-specific, weighted, averaged flash metering:
With the control process below, the subject area to be metered for flash is
specified, added, and averaged to determine the main flash output. Unlike
the EOS 10D, evaluative metering of ambient light is executed regardless of
the metering mode currently set.
1. Press the shutter button completely, then the ambient light is metered
right before the preflash is fired.
2. A preflash is fired and the metering sensor meters the entire scene.
3. For each metering zone, the ambient light reading taken in step 1 is
compared with the preflash meter reading. The area having a large
difference between the ambient light reading and preflash meter reading
is selected. (If there are more than one such area, the adjacent areas are
also selected as the flash metering area.)
4. The selected area's preflash meter reading is added and averaged. It is
compared with the ambient light reading taken in step 1, and the output
of the main flash is calculated and recorded in memory.
Since the main subject and all other objects at the same distance will be
added and averaged for the flash metering, the flash meter reading will not
change radically even if there is a big change in the position, reflectance, and
size of the main subject. The flash metering remains highly accurate and
stable.
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