What is the purpose of the "A-DEP" shooting mode on Canon cameras, for example the T3i/600D? In what kind of situations is it helpful - for example, under low light conditions, higher ISO or when using a lower shutter speed?
The purpose of A-DEP is explained in the manual (page 101):
Objects in the foreground and background will be in focus automatically. All the AF points will function to detect the subject, and the aperture required to attain the necessary depth of field will be set automatically.
Or in other words, it's useful when you don't know what aperture to set in order to get everything in focus - it lets the camera analyze the scene, pick out the "interesting" objects and then choose the right depth of field.
That's the theory anyway. In my experience, what it tends to do is analyze the scene, pick the wrong objects and give me a photo with a wonderfully sharp tree, only slightly ruined by the blurry person standing in front of it, so I'll tend to avoid A-DEP mode along with anything else where the camera chooses the active focus points.