Canon EOS Rebel SL3 DSLR Camera with 18-55mm Lens (Black) - Manual (Page 154)

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Using the Built-in Flash
To obtain a correct flash exposure, the flash output will be automatically
controlled (autoflash) to match the set aperture. The shutter speed will be
set automatically to match the brightness of the scene ( = 176).
In low light, the main subject is exposed with the autoflash, and the
background is exposed with a slow shutter speed set automatically. The
picture comes out with the standard exposure for both the subject and
background with a touch of the atmosphere (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 camera shake.
Depth-of-Field Preview
The aperture opening (diaphragm) changes only at the moment when the
picture is taken. Otherwise, the aperture remains fully open. Therefore,
when you look at the scene through the viewfinder or on the LCD monitor,
the depth of field will look narrow.
You can check the depth of field by pressing the < 0 > or < U > button
when they are configured as described below. The range in focus can be
clearly seen on Live View images as you adjust the aperture value while
holding down the < 0 > or < U > button.
- In [ 5 : Custom Functions(C.Fn)] , set [Assign SET button] to [7:Depth-
of-field preview] .
- In [ 5 : Custom Functions(C.Fn)] , set [DISP button function] to
[1:Depth-of-field preview] .