What is the difference between DEP and A-DEP modes?

DEP stands for “depth of field automatic exposure” and A-DEP stands for “automatic depth of field AE”. Both modes will choose a shutter speed and aperture combination to let you achieve a certain depth of field effect, but they do so differently. Most EOS cameras have either DEP or A-DEP modes. However one model, the 10/10s, has both and another, the EOS 1D mark II, has neither.

DEP.
To use DEP, first autofocus on a foreground item within your desired depth of field by selecting the subject and pressing the shutter halfway. “dEP 1” will appear in the viewfinder. Then recompose the image and autofocus on a background item by selecting the subject and pressing the shutter halfway. “dEP 2” will appear in the viewfinder. Finally, compose the final image in the viewfinder and press the shutter release halfway again.

The camera will then calculate the necessary aperture setting and shutter speed to keep both items, and everything in between, in focus. If this isn’t possible then the camera will blink a warning. If your camera has multiple focus points do not change the selected focus mark at any stage during this process. Press the shutter release all the way to take the photo.

A-DEP.
A-DEP requires multiple focus points and so is never available on any EOS camera with only one focus point. In this mode you arrange your image in the viewfinder such that a foreground item within your desired depth of field is covered by either the left or the right focus mark, and that a background item is covered by one of the two remaining focus marks. Press the shutter halfway and hopefully two focus marks will light up in the viewfinder telling you which items were chosen.

The camera tries to set the aperture and shutter speed such that everything between your two selected points is in focus. If it’s not possible for that to happen then the camera will blink a warning at you. If it is possible then neither the aperture nor the shutter speed will blink and you can press the shutter all the way to take the photo. A-DEP, as its name implies, is more automated and also affords less control than DEP.