[jester]

Да, лучше разобраться с этими режимами. Это кстати поможет ответить и на вопрос о сьемке с близкого расстояния. Читать вот это:
http://photonotes.org/articles/eos-flash/
Что важно:
In Av, night and Tv (shutter speed priority) modes the camera meters for ambient (existing) light and fills in the foreground subject using the flash. It does not assume that the primary light source is the flash, and therefore the shutter speed it sets is the same as it would set if you weren’t using flash at all.

Alternatively you can switch to full auto (green rectangle) or Program (P) mode, which automatically expose for the flash-illuminated subject and not the background. These modes try to ensure that the shutter speed is high enough to let you handhold the camera without a tripod. The drawback of P and basic modes is that photos taken in dimly lit areas usually end up with black or poorly lit backgrounds.

Во второй части этой статьи подробно расписано, как камера ведет себя в каждом из режимов.