Most digital cameras offer a variety of automatic exposure modes, including program, shutter priority, and aperture priority, as well as manual mode. Choosing an exposure mode determines what exposure settings you can select and what exposure settings the camera automatically selects based on other choices you have made.
In P or program mode, the camera automatically chooses both shutter speed and aperture settings. When you select the S or shutter priority mode (also called Tv for time value), you choose a shutter speed, and then the camera automatically chooses the
aperture setting to get a good exposure. Select a shutter speed appropriate to the subject — a fast speed such as 1/500 second for action or a slow shutter speed such as 1/2 second for a blur effect.
In the A or aperture priority mode (also called Av for aperture value), you choose the aperture setting that you want, and the camera selects the appropriate shutter speed. Choose a small aperture such as f/16 for deep sharpness or depth of field, and a large aperture such as f/4 for shallow or selective focus effects.
When you want complete control over both shutter speed and aperture, choose the manual mode.
Program-shooting modes (also called scene modes) such as Landscape, Macro, and Portrait offered by many digital cameras automate the process and often result in a good photograph. However, they are not likely to produce photos as good as you can get if you understand and correctly use the shutter priority or aperture priority mode settings.
Many program-shooting modes allow the camera to automatically change the ISO setting if the metering system thinks a change is needed. Sports mode, for example, shifts to a higher ISO if it needs a faster shutter speed. When the camera selects a higher ISO speed, there is potential for more digital noise in the image. If you do not want to have excess digital noise, make sure that you know when to avoid using a mode that causes automatic ISO speed changes.