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When I run my photo tours, I often get asked what exposure reading I'm getting. Rather than share with my participants the meter reading in my camera, I gently answer, "Why?" The standard retort is they want the information to make sure they're getting a proper exposure. At that point, I nod my head in agreement and answer, "It all depends."
The reason I do this is to get every photographer with whom I work to think about what it is they want as an end result. If I'm photographing a waterfall and I want to freeze the motion of every drop cascading down the rocks, I'm going to choose a shutter speed / aperture combination that allows me to shoot at least 1/500th of a second. On the other hand, if I want the same waterfall rendered with a creamy, cotton-candy effect, I'll shoot it at a shutter speed of at least one second or longer. With this in mind, my exposure reading may be totally different than someone else's.
Many photographers shoot with their cameras set to Program. I have nothing against this mode, as it provides speed, efficiency, accurate exposures and ease. But the problem with Program is that the camera makes all the choices. It doesn't know you want to pan the action to create a sharp subject against a blurred background. The ideal shutter speed for this may be 1/30th of a second, but the camera may make the exposure at 1/500th negating the effect of the pan. In aperture priority, simply rotate the aperture until a 1/30th of a second shutter appears. This guarantees you 1/30 of a second.
Conversely, there are times when specific apertures are more ideal than those randomly selected by the camera's computer in Program mode. For instance, for a portrait it's desirable to have the subject offset against an out of focus background. If the computer generated aperture in Program is f11, chances are the background will be rendered in sharp focus. To prevent this, rotate the aperture to ƒ/4 or your widest opening to limit the depth of field as much as possible. The overall goal is to create images the way you WANT them to appear so get with the program and get off Program!