Originally Posted by Colin
I think you're talking about A-DEP, automatic depth-of-field. Distance to all focus points is compared, and the camera selects an aperture that will keep all points in focus, then selects a shutter speed to work right.
P is Program. It's a manufacturer-programmed routine of aperture and shutter speed combinations based on the amount of light coming in. I don't see a chart in the 40D manual, but in my 1D Mark III manual (see Bryan's review, below the final text of the review, for a link to the PDF version) on page 189, there's a chart of how it works. Basically, in very low light, it uses the selected ISO and a wide-open aperture. Assuming you have an f/1.4 lens, at 1/50th of a second it becomes a linear progression: it goes faster by 1/3 stop, then shrinks the aperture opening by 1/3 stop, progressively until 1/8000th and f/18, and then proceeds to stop down the lens further if possible. I assume that the program picks up wherever the lens' maximum aperture lands. All in all, it's a reasonable routine for novice users (or users who are novice to a particular camera) that balances reasonable safety (camera shake is "just as likely" as too-thin DoF errors) with ease of operation. It usually allows "Program Override" - spinning the "Main Dial" or "Quick Control Dial" will take control (until the 6-second or 16-second timer expires, can't remember which).
I usually put cameras in P when my girlfriend or others are going to be using them. I just wish ISO Safety Shift was on more cameras and didn't appear to sacrifice so much buffer.
Realize though that P can be "dangerous" with flash, trending towards a "snapshot" or P&S look.
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