Canon EOS Rebel XTi Instruction Manual page 70

Av:
Changing the Depth of Field
The flash exposure will be set automatically to match the aperture that
was set. The shutter speed will set automatically between 30 sec. - 1/
200 sec. to suit the scene's brightness.
In low light, the main subject is exposed with the automatic flash, and
the background is exposed with a slow shutter speed set automatically.
Both the subject and background look properly exposed (automatic
slow-speed flash sync). If you are handholding the camera, keep it
steady to prevent camera shake. Using a tripod is recommended.
To prevent slow-speed sync, access the
[fT2
Custom Function
(C.Fn)] menu's [Flash sync. speed in Av mode] (p.1 04) and set it to
[1/200sec. (fixed)].
Press the depth-of-field preview button
to stop down to the current aperture
setting. You can check the depth of field
(range of acceptable focus) through the
viewfinder.
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