Canon EOS rebel xs - Manual (Page 74)

f : Changing the Depth of Field
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To obtain a correct flash exposure, the flash output will be set
automatically to match the set aperture (autoflash exposure). The
shutter speed will be set automatically between 1/200 sec. - 30 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, select the [7 Custom Functions (C.Fn)]
menu’s [Flash sync. speed in Av mode] (p.154) and set it to [1: 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.
Using the Built-in Flash
Depth-of-Field PreviewN
If it is difficult to discern the depth of field, first set the lowest f-number. Then
hold down the depth-of-field preview button and look through the viewfinder
while turning the <6> dial to the right. As the aperture gets smaller, the
viewfinder will gradually look darker. You can also see how the depth of field
increases.