Canon EOS-3 Instructions Manual page 119

The selectable focusing points are limited to 11. This makes focusing
point selection faster and links spot metering to the focusing point you
select. This setting is effective for spotlighted subjects on-stage, etc.,
when you want to maintain the framing of the subject.
Compared to setting 0, this setting makes focusing point selection faster.
(When automatic focusing point selection is used with settings 1 and 2, all
45 focusing points will be subject to automatic selection.)
Natural-looking fill-in flash effects are obtained automatically.
Preventing automatic reduction of flash output is effective for backlit
subjects in front of a sunset, etc., to prevent underexposure of the
subject.
By using a slow sync speed, you can create a light trail following a moving
subject.
This setting is convenient if your EX-series Speedlite does not have a
second-curtain synchronization feature. If your EX-series Speedlite is set
for second-curtain synchronization, it will override the C.Fn-15's setting.
Remarks
Custom Function Settings
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