Q: I use a Canon EOS Rebel XSi and have found that at night I seem to get more depth of field than I do in daylight. Is that possible? This photo was made at f/3.5 and 30 sec with a 18–55mm f/3.5–5.6 Canon kit lens at 18mm. How do you account for the incredible DOF at such a large aperture?
A: Depth of field isn’t affected by light levels. But it is by focal length. Your wide angle of view gives the appearance of deeper zones of sharpness, which can seem even deeper if you focus at or near the lens’ hyperfocal distance.
(from Tech Support June 2009)