When you’re using a Creative Zone mode, you’ll have to judge for yourself when flash might be useful, and flip it up yourself by pressing the Flash button on the side of the pentaprism. The behavior of the internal flash varies, depending on which Creative Zone mode you’re using.
P/A-DEP. In these modes, the XSi fully automates the exposure process, giving you subtle fill flash effects in daylight, and fully illuminating your subject under dimmer lighting conditions. The camera selects a shutter speed from 1/60th to 1/200th second and sets an appropriate aperture.
Av. In Aperture Priority mode, you set the aperture as always, and the XSi chooses a shutter speed from 30 seconds to 1/200th second. Use this mode with care,