Getting Comfortable with Digital Exposure Controls

Because light levels vary, digital cameras must vary the amount of light that reaches the sensor. One way to do that is to change the f-stop. The second way is to alter the length 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 — the shutter speed.

Like f-stops, 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, 1/50, and 1/1000 of a second. However, your digital camera readout might show them as 60, 125, 250, 500, or 1000 because the numerator is always 1.

When shopping for a digital camera, you'll want to look for the exposure options described in the following sections.

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 (EVs 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.

Full autoexposure

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.

Aperture-preferred/shutter-preferred exposure

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 — commonly, Av and Tv (Time value). Figure 1 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 for:

• Sensor sensitivity: Like film, sensors have varying degrees of sensitivity to light. (For film, this is its speed.) 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 specs often use that terminology.

Unfortunately, upping the ISO rating usually increases the amount of random fuzziness — noise — in the image. If you plan to shoot many pictures in very dim light, any camera you buy should be tested first to see whether ISO settings can be set manually. Then check how noisy the pictures become when you increase the sensitivity.

• 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); sometimes, 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. Measuring a center spot sometimes 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. Or the camera might have an override to the exposure system, allowing the sensor to receive more light from backlit subjects when you choose to activate it.

• Manual exposure: If you're seriously interested in photography, you'll want at least the option of setting exposure (both f-stop and shutter speed) manually so that you can custom-tailor your exposure to the artistic effect you're trying to achieve.