Canon eos rebel T2i - Manual (Page 82)

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.
If you do not want a slow shutter speed to be used, set the [7 Custom
Functions (C.Fn)] menu’s [Flash sync. speed in Av mode] to [1: 1/
200-1/60 sec. auto] or [2: 1/200 sec. (fixed)] (p.192).
Press the depth-of-field preview button
to stop down to the lens’s current
aperture setting. You can then check the
depth of field (range of acceptable focus)
through the viewfinder.
Using the Built-in Flash
Depth of Field PreviewN
The depth-of-field effect can be clearly seen on the Live View image as you
change the aperture and press the depth-of-field preview button (p.112).
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