Canon EOS Rebel XT - Manual (Page 96)

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E-TTL II autoflash obtains high-precision and consistent flash shots.
If necessary, the built-in flash will pop-up automatically in low-light
or backlit conditions. (except in the <
3> <5> <7> modes)
Regardless of the light level, you can press the <
D> button to pop-up
and fire the built-in flash whenever desired.
d
: For fully automatic flash photography. The shutter speed (1/
60 sec. - 1/200 sec.) and aperture value are set automatically,
just as in <
1> (Full Auto) mode.
s : Enables you to set the desired shutter speed (30 sec. - 1/
200 sec.). The camera then automatically sets the flash
aperture value to obtain the proper exposure at the shutter
speed you have set.
f : Enables you to set the desired aperture value. The camera
then automatically sets the shutter speed (30 sec. - 1/200
sec.) to obtain the proper exposure at the aperture you have
set.
Against dark backgrounds such as the night scenes, slow-sync
shooting will be set so that both the subject and background are
exposed correctly. The main subject is exposed with the flash,
and the background is exposured with a slow shutter speed.
¡ Because automatic slow-sync shooting uses a slow shutter
speed, always use a tripod.
¡ If you do not want a slow shutter speed to be set, set C.Fn-3
[Flash sync. speed in Av mode] to [1: 1/200sec. (fixed)].
(p.148)
a : You can set both the shutter speed (bulb or 30 sec. - 1/200
sec.) and aperture value. The main subject is exposed
properly by the flash. The background exposure will vary
depending on the shutter speed and aperture.
8: The flash result will be the same as the <d> mode.
Using the Built-in Flash
Using the Built-in Flash in the Basic Zone
Using the Built-in Flash in the Creative Zone