Canon EOS 20D - Manual (Page 92)

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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 <
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. - 1/250 sec.) and aperture value are set automatically,
just as in <
1> (Full Auto) mode.
s : For when you want to set the shutter speed (30 sec. - 1/250
sec.). The camera then automatically sets the flash aperture
value to provide the proper exposure for your shutter speed.
f : For when you want to set the aperture value. The camera
then automatically sets the shutter speed (30 sec. - 1/250
sec.) to provide the proper exposure for your aperture. Against
dark backgrounds such as the night sky, 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-03
[Flash sync speed in Av mode] to [1: 1/250sec. (fixed)].
(p.141)
a : You can set both the shutter speed (bulb or 30 sec. - 1/250
sec.) and aperture value. The main subject is exposed
properly by the flash. The background exposure varies
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