P is "Program auto exposure" where you choose the ISO, while the camera chooses both what it "thinks" are an appropriate lens aperture (f-stop) and an appropriate shutter speed to make a "correct" exposure. You get to make changes to other camera settings, such as choosing white balance, selecting the type of file to save, adjusting auto focus as you see fit for the situation, tweaking exposure with Exposure Compensation, etc.
Full "Auto" is basically a "point n shoot" mode that not only uses the P auto exposure (AE) mode as described above, but it also overrides and dictates everything else listed and more. You cannot make any of those other choices. The camera will only use Auto White Balance, only save large/fine JPEG files, force you to use the most automated auto focus mode, won't let you dial in any Exposure Compensation, etc. Think of it as a "SUPER Auto" mode... way more than just auto exposure and far from the same as "P".
Honestly, I cannot recall ever using full Auto (sometimes "A+") on any DSLR I've used the past 14 or 15 years. I have a couple compact P&S cameras where I've occasionally used it for "snapshots".
I do very occasionally use P on my DSLRs... Usually only when shooting very fast and I don't have time to give the settings much thought. Often this is when I'm switching modes to take a few shots under radically different lighting, then switching back.
But far more often I use either...
1. Av (also A on some cameras) "aperture priority" AE, where I select the aperture and ISO, the camera chooses a shutter speed. Useful when I want to control depth of field in images.
2. Or Tv (S on some cameras) "shutter priority" AE where I select the shutter and ISO, the camera chooses an aperture. Useful when I want to insure a shutter speed that freezes movement or deliverately allows blur.
3. Or M (full manual, without Auto ISO) where I select everything and don't allow the camera to make any settings. Useful when I really want to lock down my exposures, avoid issues from back lighting and other things that can throw off auto exposure modes.
4. Or occasionally M with Auto ISO, which many cameras now offer. When it's combined with M this makes for yet another form of auto exposure, where I choose the shutter speed and the aperture for the above reasons, but let the camera select an ISO. This is probably the most misunderstood and most overused mode now. I've just been experimenting with it for a year or so (first time I've had cameras where it's fully implemented and usable). Often folks use Auto ISO with M and think they're "only shooting manual", as if that's something special (it's not
). They're just kidding themselves though, because they aren't really shooting manual at all.... they are just using a relatively new form of auto exposure. (Further.... I'd never combine Auto ISO with P, Av/A or Tv/S... because with Auto ISO any of those would become a rather unpredictable "double AE" mode... and I really can't think of any situation where such a combo might be purposeful and helpful).
Hope this helps!
P is "Program auto exposure" where you c... (