When do you use which setting?
It depends on the situation, what is important for you. Do you want to set the depth of field or would you like to control the shutter speed? Or both?
P stands for Program. It is a setting that gives you an aperture and a shutter speed when you press the shutter button. The values depend on the available light. When you half-press the shutter button you see them and when you turn the wheel, both shutter speed and aperture go in a different direction. When the shutter speed becomes faster (less light), the aperture number becomes lower (more light). When you dial in the opposite direction, your aperture number becomes bigger (for a bigger depth of field) and as a reaction the shutter speed will become slower. The camera is programmed to give you a certain amount of light, so if you change one, the other one will change too.
It is almost automatic, but you can still change your ISO settings and certain other settings become available. One handy thing is you can switch of the dreaded flash. It operates within the boundaries of your aperture range. So really fast or slow shutter speeds may not be possible.
You choose the aperture, the camera chooses the shutter speed.
You are in control of the depth of field. This is really important, as here you can choose to only have your subject sharp or a bigger part of the photo (or even everything). It is not necessary to have everything sharp. It all depends on what you like.
With aperture you can really tell the story you want to tell. If only your subject is sharp it is very clear what or who your subject is. A low number is a shallow depth of field, a higher number is a bigger depth of field. You would want a higher number if you want to show more of the scene. This is handy for when you want to include other things into the photo, to tell something about the subject or area.
In many situations the A setting works. But remember that the camera will give you a shutter speed and it may be too slow to freeze the moment. If you’re into action shots, A is not your mode. If you want to be certain to nail the moment, A is not your mode. Most people who know more about settings shoot in aperture priority. Personally I don’t. For me it is essential to have the shutter speed I need and only I can set it. If I do find aperture important I will go straight to manual (M).
Be very careful with aperture priority. If you miss the moment because your shutter speed is too slow there is no second chance.
You choose the shutter speed, the camera chooses the aperture.
Only when you control the shutter speed you can have sharp photos when your subject moves faster or really fast (action shots – sports, children).
The faster the action, the faster the shutter speed should be. That may be 1/200th of a second, 1/400th, 1/800th or even faster. If your shutter speed is not fast enough you may have motion blur. You may miss the moment.
With slower shutter speeds you create motion and you can also eliminate distraction in your composition. When something is not sharp, it does not stand out anymore.
You set both the shutter speed and aperture. If you want to have full control you set both manually. You know what you want, the camera doesn’t.
When using manual you need more time to choose the correct settings. Light changes all the time, so often you need to adjust your settings. Unless you control the light, as in a studio, or when you stay for a long time with your subject.
If you want a faster shutter speed AND a bigger depth of field, manual is the way to go. With the A, S and P settings this may not be possible. With M you have maximum control, which is good to get exactly what you want or when the light remains consistent.
Shooting in the manual mode can be deadly when you move around all the time, looking for interesting moments. When you move from a darker area into a brighter area, and you don’t change your settings, there is a high chance your photo will be over-exposed.
So if you need flexibility, M is not your choice. If you travel with family or friends, they will all hate you when you spend all your time on the settings and not on them.
Some brands like Canon and one or two more, use different letters for S and A: TV (Time Value) and AV (Aperture Value).