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P lets the camera sort out aperture (wideness of hole the light's coming through) and shutter speed (how long the hole stays open) for you - by doing that you cede control, but the camera will keep the values within it's capabilities.Originally Posted by Rob_B
A gives you control of aperture. You set the size of the hole, and the camera then works out how long it needs to stay open to properly expose the scene.
S does the same but for shutter speed - you set it, and the camera works out how wide the hole must open for the scene to be properly exposed by the time it's closed again.
You don't need to 'know' the exposure parameters - the camera will have a light meter built in and in all three modes it will adjust settings to properly expose the scene. The exceptions only occur when you select such extreme values that the camera doesn't have the range of adjustment to create the right exposure, for example setting an extremely fast shutter speed with a lens that doesn't open very wide.
Adjusting the shutter speed has the most noticeable effect with a compact/bridge camera, as it's the usual blur thing. Adjusting the aperture might have a slight effect on the depth of field (how big the region in focus is), but with these camera's it's not a huge effect. Rather it's a) another tool to get the correct exposure for the automatic settings and b) it can be set to keep the aperture at the value the camera performs best - my canon 720is was sharpest at f/4 for example.
The final frontier parameter you want to look at is ISO - this is the sensitivity (gain level). You can turn this up to require less light for the same exposure, which lets you get away with a faster shutter speed or smaller aperture (ie higher f number - f/22 is much smaller than f/4 - it's 'divided by'). The problem is the higher the gain, the more noisy the image becomes.
To actually change the exposure of a scene, you either have to shoot M, or more simply, use exposure compensation, which adds a simple + or - to the calculation of the correct exposure. It's not needed most of the time, but sometimes our eyes are compensating a lot more than we realise - sunsets need some -ve compensation or the camera tries to make them into daylight. Snowy scenes need some +ve or the camera tries to make them into grey. If in doubt, take several photos with different compensation values. If you line them up you can even start exposure blending.
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