Canon EOS 30D - Manual (Page 89)

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f Aperture-Priority AE
If the “30"” shutter speed blinks, it indicates
underexposure. Turn the <6> dial to set a larger
aperture (smaller f/number) until the blinking stops or
set a higher ISO speed.
If the “8000” shutter speed blinks, it indicates
overexposure. Turn the <6> dial to set a smaller
aperture (larger f/number) until the blinking stops or set
a lower ISO speed.
Aperture Display
The larger the f/number, the smaller the aperture opening will be. The
apertures displayed will differ depending on the lens. If no lens is attached to
the camera, “00” will be displayed for the aperture.
Press the depth-of-field preview button to
stop down to the current aperture setting.
The diaphragm in the lens will be set to
the current aperture so you can check the
depth of field (range of acceptable focus)
through the viewfinder.
Depth of Field PreviewN
A higher f/number will make more of the foreground and background fall
within acceptable focus. However, the viewfinder will look darker.
In the <8> mode, press the shutter button halfway to focus, then
press the depth-of-field preview button while still pressing the shutter
button halfway.
The exposure is locked (AE lock) while the Depth-of-Field Preview
button is pressed.