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The brightness or darkness of a photographic image depends on exposure — the amount of light that hits the film or image-sensor array. The more light, the brighter the image. Too much light results in a washed-out, or overexposed, image; too little light, and the image is dark, or underexposed.
Most low-priced digital cameras don't give you much control over exposure; everything is automatic. But many midrange cameras offer a choice of automatic exposure settings, and higher-end cameras typically provide manual exposure control in addition to automatic options.
Several factors affect exposure — including shutter speed, aperture, and ISO (International Standards Organization) settings — and they influence the limitations and possibilities of your camera.
The film perspective
Before taking a look at how digital cameras control exposure, it helps to understand how a film camera does the job. Even though digital cameras don't function in quite the same way as a film camera, manufacturers describe their exposure control mechanisms by using traditional film terms.
Figure 1 shows a simple illustration of a film camera. Although the specific component design varies depending on the type of camera, all film cameras include some sort of shutter, which is placed between the film and the lens. When the camera isn't in use, the shutter is closed, preventing light from reaching the film. When you take a picture, the shutter opens, and light hits the film.
Figure 1: A look at the shutter and aperture in a traditional film camera.
You can control the amount of light that reaches the film in two ways: by adjusting the amount of time the shutter stays open (referred to as the shutter speed) and by changing the aperture. The aperture is a hole in an adjustable diaphragm set between the lens and the shutter. Light coming through the lens is funneled through this hole to the shutter and then onto the film. So if you want more light to strike the film, you make the aperture bigger; if you want less light, you make the aperture smaller.
The size of the aperture opening is measured in f-numbers, more commonly referred to as f-stops. Standard aperture settings are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22.
 | Contrary to what you may expect, the larger the f-stop number, the smaller the aperture and the less light that enters the camera. Each f-stop setting lets in half as much light as the next smaller f-stop number. For example, the camera gets twice as much light at f/11 as it does at f/16 (see Figure 2). |
Figure 2: As the f-stop number decreases, the aperture size grows and more light enters the camera.
Shutter speeds are measured in more obvious terms: fractions of a second. A shutter speed of 1/8, for example, means that the shutter opens for one-eighth of a second. Try to capture a moving object at that speed and you wind up with a big blur. You need a shutter speed of about 1/500 to capture action clearly.
On cameras that offer aperture and shutter speed control, you manipulate the two settings in tandem to capture just the right amount of light. For example, if you're capturing fast action on a bright, sunny day, you can combine a fast shutter speed with a small aperture (high f-stop number). To shoot the same picture at twilight, you need a wide-open aperture (small f-stop number) in order to use the same fast shutter speed.
The digital perspective
As with a film camera, the exposure of a picture shot with a digital camera depends on the amount of light that the camera captures. But some digital cameras don't use a traditional shutter/aperture arrangement to control exposure. Instead, the chips in the image-sensor array simply turn on and off for different periods of time, thereby capturing more or less light. In some cameras, exposure is also varied — either automatically or by some control the user sets — by boosting or reducing the strength of the electrical charge that a chip emits in response to a certain amount of light.
Even on cameras that use this alternative approach to exposure control, the camera's capabilities are usually stated in traditional film-camera terms. For example, you may have a choice of two exposure settings, which may be labeled with icons that look like the aperture openings shown in Figure 2. The settings are engineered to deliver the equivalent exposure that you would get with a film camera using the same f-stop.
ISO ratings and chip sensitivity
 | Aperture and shutter speeds aren't the only factors involved in image exposure. The sensitivity of the image-sensor array also plays a role. Pick up a box of film, and you should see an ISO number. This number tells you how sensitive the film is to light and is also referred to as the film speed. |
Film for the consumer market typically offers ratings of ISO 100, 200, or 400. The higher the number, the more sensitive (or faster) the film. And the faster the film, the less light you need to capture a decent image. The advantage of using a faster film is that you can use a faster shutter speed and shoot in dimmer lighting than you can with a low-speed film. On the downside, photos shot with fast film sometimes exhibit noticeable grain —a slightly speckled appearance.
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