Choosing an Exposure Method

You’ll find four methods for choosing the appropriate shutter speed and aperture, when using the semi-automatic/manual modes. (Scene modes, which use their own exposure biases, are described next.) You can choose among Program, Aperture-priority, Shutter-priority, or Manual options by rotating the mode dial on top of the camera. Your choice of which is best for a given shooting situation will depend on things like your need for lots of (or less) depth-of-field, a desire to freeze action or allow motion blur, or how much noise you find acceptable in an image. Each of the D5100’s exposure methods emphasizes one aspect of image capture or another. This section introduces you to all four.

Aperture-Priority

In A mode, you specify the lens opening used, and the D5100 selects the shutter speed. Aperture-priority is especially good when you want to use a particular lens opening to achieve a desired effect. Perhaps you’d like to use the smallest f/stop possible to maximize depth-of-field in a close-up picture. Or, you might want to use a large f/stop to throw everything except your main subject out of focus, as in Figure 4.13. Maybe you’d just like to “lock in” a particular f/stop because it’s the sharpest available aperture with that lens. Or, you might prefer to use, say, f/2.8 on a lens with a maximum aperture of f/1.4, because you want the best compromise between speed and sharpness.