Setting Advanced Exposure Modes on a Nikon DSLR
In the automatic shooting modes available to you on your Nikon D3100, D5100, or D7000 you have very little control over exposure. You may be able to choose from one or two Flash modes, and you can adjust ISO in the Scene modes. But to gain full control over exposure, set the Mode dial to one of the advanced modes: P, S, A, or M. You also need to shoot in these modes to use certain other features, such as manual white balancing.
The major difference among the four advanced modes is the level of control over aperture and shutter speed, as follows:
P (programmed autoexposure): In this mode, the camera selects both aperture and shutter speed. But you can choose from different combinations of the two for creative flexibility.
S (shutter-priority autoexposure): In this mode, you select a shutter speed, and the camera chooses the aperture setting that produces a good exposure at your selected ISO setting.
A (aperture-priority autoexposure): The opposite of shutter-priority autoexposure, this mode asks you to select the aperture setting. The camera then selects the appropriate shutter speed to properly expose the picture.
M (manual exposure): In this mode, you specify both shutter speed and aperture.
To sum up, the first three modes are semi-automatic exposure modes that are designed to help you get a good exposure while still providing you with some photographic flexibility. You can even modify the autoexposure results by using features such as Exposure Compensation. But it's important to note that in extreme lighting conditions, the camera may not be able to select settings that will produce a good exposure, and it doesn't stop you from taking a poorly exposed photo.
Manual mode puts all exposure control in your hands. If you're a longtime photographer who comes from the days when manual exposure was the only game in town, you may prefer to stick with this mode. If it ain't broke, don't fix it, as they say. And in some ways, manual mode is simpler than the semi-auto modes because if you're not happy with the exposure, you just change the aperture, shutter speed, or ISO setting on your D3100, D5100, or D7000 and shoot again. You don't have to fiddle with features that enable you to modify your autoexposure results — although again, the lighting conditions determine whether a good exposure is possible at any combination of settings.