Canon EOS 40D - Manual (Page 53)

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To shoot someone at night and obtain a natural-looking exposure in the
background, use the <6> (Night Portrait) mode.
Use a wide-angle lens and a tripod.
If you use a zoom lens, set it to the wide-angle end to obtain a wide
night view. Use a tripod to prevent camera shake.
Keep the person within 5 meters/16.4 feet from the camera.
Under low light, the built-in flash will fire automatically to obtain a
good exposure of the person. The effective distance of the built-in
flash is 5 meters/16.4 feet from the camera.
Shoot also with <1> (Full Auto).
Since camera shake is prone to occur with night shots, shooting also
with <1> (Full Auto) is recommended.
6 Shooting Portraits at Night
Shooting Tips
If the self-timer is also used, the self-timer lamp will flash after the picture is
taken.