Because light levels vary, digital cameras must vary the amount of light reaching the sensor. One way to do that is to change the f-stop, the relative size of the aperture (opening). The second way is to alter the amount of time that the sensor is exposed to the light. This is done either electronically or with an actual mechanical device -a shutter - that opens and closes quickly to expose the sensor for a set period, known as shutter speed.
Shutter speeds are the denominators of fractions. The larger the number, the less light that reaches the sensor. Typical shutter speeds include 1/60, 1/125, 1/250, 1/500, and 1/1000 of a second. Cameras (primarily film models) using mechanical shutters may have only exact intervals like those available. Electronic shutters aren't limited to those values and can provide you with any actual interval from a few seconds through 1/2000, or sometimes 1/4000, of a second. The traditional shutter speed values are useful only when calculating equivalent exposures.
When shopping for a digital camera, look for these 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 camera's light-measuring system determines. These adjustments are called exposure values (EV for short), and most digital cameras let you fine-tune exposure +/- about 2EV.
Full autoexposure: With this option, your digital camera selects the shutter speed and lens opening for you, 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 dimmer light, the camera may 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.
Aperture-preferred/shutter-preferred exposure: These options let you choose a lens opening (aperture-preferred) or shutter speed (shutter-preferred) and then set the other control to match. These settings might be indicated by A or S markings - 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. Figure 1 shows an action shot taken with the camera set on shutter-preferred.
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. Complete control is good for creative reasons because underexposing (say, to produce a silhouette effect) may be exactly what you want.
Other factors to consider
In addition to exposure options, other factors must be considered when evaluating the exposure controls of your dream digital camera. Here is a quick checklist of those you should look for.
Sensor sensitivity: Like film, sensors have varying degrees of sensitivity to light. The more sensitive the sensor is, the better it can capture images in low light levels. Most digital cameras have a sensitivity that corresponds roughly to that of ISO 50 to ISO 100 film (so-called slow film), and the specifications often use that terminology. Many cameras let you specify the sensitivity, increasing from the default value of, say, ISO 100 to ISO 400 or even ISO 800 to give you a "faster" camera. Unfortunately, upping the ISO rating usually increases the amount of random fuzziness - called noise - in the image.
Measurement mode: Just how does your digital camera's exposure system measure the light? Sometimes, it measures only the center of the picture (which is probably your subject anyway). Other times, it might measure the entire frame and average out the light that the sensor sees. You don't always want the camera to measure the light the same way. Sometimes, measuring a center spot produces the most accurate reading. Other times, such as when the scene is evenly lit, an averaging system works best.
Compensation systems: Many exposure systems can sense when a picture is backlit (most of the light is coming from behind the subject) and add exposure to make the subject brighter. Sophisticated cameras can analyze your scene and choose an exposure mode that best fits each individual picture, compensating for potential trouble spots in the photograph.
Manual exposure: If you're seriously interested in photography, you'll want at least the option of setting exposure manually (both f-stop and shutter speed) so you can custom-tailor your exposure to the artistic effect you're trying to achieve.