Canon EOS 450D - Manual (Page 76)

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Objects in the foreground and background will be in focus automatically.
All the AF points will function to detect the subject, and the aperture
required to attain the necessary depth of field will be set automatically.
* <8> stands for Auto-Depth of field. This mode sets the depth of field
automatically.
1
Set the Mode Dial to <8>.
2
Focus the subject.
Aim the AF points over the subjects
and press the shutter button halfway.
(0)
All the subjects covered by the AF
points flashing in red will be in focus.
3
Take the picture.
The shutter speed display in the viewfinder blinks.
If the “30"” shutter speed blinks, it means that the subject is too dark.
Increase the ISO speed. If the “4000” shutter speed blinks, it means
that the subject is too bright. Decrease the ISO speed.
The aperture display in the viewfinder blinks.
The exposure is correct, but the desired depth of field cannot be
obtained. Either use a wide-angle lens or move farther away from
the subjects.
A slow shutter speed has been set.
Use a tripod.
I want to use flash.
Flash can be used, however, the result will be the same as using the
mode with flash. The desired depth of field will not be obtained.
8: Automatic Depth-of-Field AE
FAQ