Canon EOS Rebel XTi Instruction Manual page 187

Set the Mode Dial to <Av>.
Turn the
<B>
dial to set the desired
aperture, then focus the subject.
The shutter speed will be set
automatically.
If the shutter speed display blinks, turn
the
<B>
dial until it stops blinking.
(using EF-S18-55mm f/3.5-5.6 II lens)
Approx. 1-14.9m (3.3-48.9ft) Approx. 1-9.3m (3.3-30.5ft)
Approx.1-7.4m (3.3-24.3ft) Approx.1-4.6m (3.3-15.1ft)
Approx. 1-3.7m (3.3-12.1 ft) Approx. 1-2.3m (3.3-7.5 ft)
400
1600
In Basic Zone Modes
If necessary, the built-in flash will pop-up automatically in low-light
or backlit conditions. (except in the <A&>
<~>
<m> modes)
Built-in Flash
Creative Zone
Modes
Press the
<~
>
button to pop up
the built-in flash,
then shoot.