Canon EOS REBEL XS Instruction Manual page 74

AV: Changing the Depth of Field
Using the Built-in Flash
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 11200 sec. o 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 [t?_ Custom Functions (C.Fn)]
menu's [Flash synco speed in Av mode] (p.154) and set it to [1 : 1t
200see. (fixed)].
Depth.of.FieJd Preview*
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.
_f it is difficult to discern the depth O[fie!d, fii_st S etthe lowest f-number. Then
ho!d down the depth-of-fie!d preview button and look through the viewfinder
whil e turning the < _ > 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.
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