Canon DS126251 ユーザーズマニュアル

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AF Area Selection Modes
Under low-light conditions, when you press the shutter button halfway, 
the built-in flash fires a brief burst of flashes. It illuminates the subject to 
enable easier autofocusing.
With lenses whose maximum aperture is larger than f/5.6
With all AF points, cross-type AF sensitive to both vertical and 
horizontal lines is possible.
With lenses whose maximum aperture is larger than f/2.8*
With the center AF point, high-precision, cross-type AF sensitive to both 
vertical and horizontal lines is possible. The center AF point’s sensitivity 
to vertical and horizontal lines is about twice as sensitive as the other 
AF points.
The remaining 18 AF points will work as cross-type points with lenses 
brighter than f/5.6.
* Except with the EF28-80mm f/2.8-4L USM and EF50mm f/2.5 Compact Macro lenses.
AF-Assist Beam with the Built-in Flash
Lens’ Maximum Aperture and AF Sensitivity
 
If [8C.Fn III -12: Orientation linked AF point] is set to [1: Select 
different AF points
], you can set the AF area selection mode and 
manually-selected AF point (or zone in the case of Zone AF) separately 
for vertical and horizontal shooting (p.214).
 
If [8C.Fn III -10: Focus display in AI SERVO/MF] is set to [Disable], 
the active AF points <S> during AI Servo AF will not be displayed.
 
The built-in flash’s AF-assist beam is effective up to about 4 meters/13.1 
feet.
 
In the d/s/f/a/F shooting modes, press the <I> button to pop-
up the built-in flash. Then it will fire the AF-assist beam when necessary.
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