Canon EOS 40D - Manual (Page 54)

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In places where flash photography is prohibited, use the <7> (Flash
Off) mode. This mode is also effective for candlelight scenes when you
want to obtain the candlelight effect.
If the numeric display in the viewfinder blinks, take care to
prevent camera shake.
Under low light when camera shake is prone to occur, the
viewfinder’s shutter speed display will blink. Hold the camera steady
or use a tripod. If you have a zoom lens, use the wide-angle end to
reduce blur due to camera shake.
Taking portraits without flash.
Under low light, the person must not move until the picture is taken.
If the person moves during the exposure, he or she might look
blurred in the picture.
7 Disabling Flash
Shooting Tips