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For example, suppose a basic exposure were 1/250 of a second at f/16 (which is a typical exposure for a digital camera in bright daylight). If you wanted to stop some fast-moving action, you might want your camera to switch to 1/500 of a second to freeze the movement with the shorter exposure time. (You can find out more about stopping action in Book IV, Chapter 5.)
Because youíve cut the amount of light in half by reducing the shutter speed from 1/250 to 1/500 of a second, your camera needs to compensate by doubling the amount of light admitted through the lens. In this example, the lens is opened up from f/16 to f/11, which lets in twice as much illumination.
Usually, all this happens automatically, thanks to your digital cameraís handy autoexposure modes, but you can set these controls manually on some models if you want. Itís probably a better idea to choose a different autoexposure mode (described later) that gives you the flexibility you need. Sometimes, youíll look at an image on your LCD review screen and decide that the picture is too dark or too light. A digital cameraís autoexposure modes can take care of this, too. When shopping for a digital camera, youíll want to look for the following exposure options.
Plus/minus or over/under exposure
With these modes, you can specify a little more or a little less exposure than the ideal exposure that your cameraís light-measuring system determines. These adjustments are called exposure values (EV for short), and most digital cameras let you fine tune exposure +/- about 2EV, using half- or third-stop increments. The most conveniently designed cameras have the EV adjustment available from one of the main buttons on the camera, such as the Up-cursor key. Beware of cameras that make you wend your way into the menu system to make an EV adjustment. Fortunately, after itís set, the EV setting ďsticksĒ so that you can continue to take pictures in the same environment with the modified setting.
With this option, your digital camera selects the shutter speed and lens opening for you by using built-in algorithms, called programs, that allow it to make some intelligent guesses about the best combination of settings.
For example, on bright days outdoors, the camera probably chooses a short shutter speed and small f-stop to give you the best sharpness. Outdoors in
Understanding How Lenses Work 95
dimmer light, the camera might select a wider lens opening while keeping the shutter speed the same until it decides to drop down to a slower speed to keep more of your image in sharp focus.
Some digital cameras have several autoexposure program modes (sometimes called scene settings) to select from, so if youíre taking action pictures and have chosen an action-stopping mode, the camera tries to use brief, action-stopping shutter speeds under as many conditions as possible. These program modes are different from the aperture/shutter-preferred modes described next because they frequently include other factors in their adjustments. They might have names like Sports, Night, Night Portrait, Landscape, Fireworks, or Portrait. I used a camera with a scene setting called Cuisine (I kid you not), which (after a little digging) I discovered also increased the sharpness and color richness of the picture to supposedly make food pictures look better. Some cameras have both an Auto (A) and Program (P) setting.
Book II Chapter 1
These options let you choose a lens opening (aperture-preferred) or shutter speed (shutter-preferred), and then the camera automatically sets the other control to match. These settings might be indicated by A or S markings ó commonly, Av and Tv (Time value). For example, by choosing shutter-preferred, you can select a short shutter speed, such as 1/1000 of a second, and the camera locks that in, varying only the f-stop.
Unlike the programmed modes described in the preceding section, if your camera finds that the selected shutter speed, for example, canít be mated with an appropriate aperture (itís too dark or too light out), it might not take a photo at all. Iíve run into this at soccer games when Iíve set my camera to an action-stopping shutter speed, but clouds dim the field so much that even the largest lens opening isnít enough to take a picture. Figure 1-6 shows an action shot taken with the camera set on shutter priority.
Full manual control
With this option, you can set any shutter speed or aperture combination you like, giving you complete control over the exposure of your photo. That means you can also completely ruin the picture by making it way too dark or much too light. However, complete control is good for creative reasons because seriously underexposing (say, to produce a silhouette effect) might be exactly what you want.
Other factors to consider
In addition to exposure options themselves, you must consider other factors when evaluating the exposure controls of your dream digital camera. Here is a quick checklist of those you should look out for: