ebiggs and cicopo are both basically giving you very similar advice. There's a subtle difference between the two modes.
In full auto mode (green A+ box on the mode dial) the camera is fully automatic. It will set the exposure and it will not let you override it. In fact, it wont let you override much of anything. That mode tries to be somewhat "fool proof" in that if you mess with buttons on the camera body -- many of the button presses will be ignored in full auto mode.
Program mode ("P" on the mode dial) is siimilar in that it also works out the exposure exactly the way it would have in fully automatic mode. But the difference is that the "P" mode will actually let you override some of the settings. Given some amount of light... you can either take a short exposrue with the camera allowing a lot of light thorugh the lens (but only very briefly) -OR- you can take a longer exposure duration... but with the camera only allowing a small amount of light through the lens. Ultimately the same total of amout of light is collected and you might think these would result in identical images. But it turns out "fast" images tend to freeze action, but have a shallow "depth of field" (this is the range of distances at which things will appear to be more or less acceptably focused.) When the camera uses a smaller aperture opening (to allow in less light) the depth of field grows dramatically -- so LOTS of stuff is in focus (great for landscape photos)... but since the shutter needs to be open longer, it's not good for freezing action (and you may even need a tripod if the exposure time is long enough.)
That last paragraph is meant to say that the creative result of the image captured by the camera will actually be different. This si THE REASON why photographers want to learn how to leave full "automatic" mode and learn to control those parts of the exposure that alter the creative results.
If I take a photo of a waterfall, I might want to put the camera on a tripod and use a setting which allows me to use a very long shutter duration so the motion of the water is blurred (but everything else is tack-sharp.) This creates those gorgeous milky / dreamy looking waterfalls. If I want to freeze action -- say... birds in flight -- then I'll use a much faster shutter speed so that my bird isn't blurred. This is what I mean by "creative" results. Full auto mode wont ask you to make these choices. It'll just pick a "middle of the road" safe exposure.
The easiest mode on the dial is the fully automatic green "A+" position. But it wont let you change much of anything.
The next step toward taking control is the Program mode ("P" on the mode dial) which will let you override settings (and it will let you override things even to the point of ruining the shot -- but you do have to deliberately override the camera's exposure advice and this is not likely to happen by accident.)
Tim Campbell
5D III, 5D IV, 60Da
