Canon EOS 40D - Manual (Page 99)

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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. If the built-in flash has
popped up, you can push it back down with your fingers.
d
: For fully automatic flash photography. The shutter speed
(1/ 60 sec. - 1/250 sec.) and aperture are set automatically.
s : Enables you to set the desired shutter speed (30 sec. - 1/
250 sec.). The flash exposure will be set automatically to match
the aperture that was set automatically.
f : Enables you to set the desired aperture. The flash exposure
will be set automatically to match the aperture that was set. The
shutter speed will set automatically between 30 sec. - 1/250
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).
With slow shutter speeds, using a tripod is recommended.
If you do not want to use a slow shutter speed, set [8C.Fn I
-7] (Flash sync. speed in Av mode) to [1:1/250sec. (fixed)].
(p.155)
a : You can set both the shutter speed (bulb or 30 sec. - 1/250
sec.) and aperture. The flash exposure will be set automatically
to match the aperture that was set. The background exposure
will vary depending on the shutter speed and aperture.
8 : The flash result will be the same as the mode.
Using the Built-in Flash
Using the Built-in Flash in the Basic Zone
Using the Built-in Flash in the Creative Zone