Canon EOS 20D Service Manual page 89

If the difference between the ambient light reading and preflash reading is
extremely large, it is likely that there is a highly reflective object. This
reflective area is eliminated and the preflash reading is adjusted to a lower
reading to prevent underexposure.
Incorporating lens distance information:
When a lens having distance information is used, the flash exposure will be
even more stable for flash pictures which have a highly reflective object or
white clothing worn by the subject.
If there is a highly reflective object in the scene, there will be an extremely big
difference between the ambient light reading and preflash reading in only one
area. If the lens provides distance information, the distance information is
also incorporated to determine whether there is a highly reflective object. This
further ensures that the highly reflective object is eliminated from the final
exposure. (If the lens does not provide distance information, the highly
reflective object's adverse effect on the exposure is reduced based on the
available information.)
Also, since a highly reflective object tends to result in an underexposed
shot, the distance information is also incorporated to reduce the degree of
underexposure.
Since the distance information is zone information for *** to *** meters, it is
used as a supplemental method to stabilize flash exposures for such atypical
scenes.
Note: For macrophotography, the distance information is not used.
2) Exposure control system
As with the EOS 10D, the EOS 20D has eleven AE modes and a manual mode.
When A-DEP (automatic depth-of-field AE) is used, AF is executed with nine AF
points, the optimum aperture is set, and the shutter speed is set automatically.
3) Shutter
A newly-developed shutter dedicated to the APS-C size is employed. By making
the shutter dedicated to this size, compactness, high speed, and high durability
are attained.
Table 007 shows the shutter design specifications.
(1) APS-C-size design
Since the shutter opening is the same size as the
APS-C size, the shutter blades are smaller. This
minimizes the inertial mass so that high-speed
shutter speeds can be obtained with a little force.
Also, having smaller shutter blades makes the
shutter unit smaller.
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Technical Information
Fig. 027 Shutter unit