You, or the camera's automatic exposure system, control exposure using the camera's shutter speed and aperture. Both affect the exposure, the total amount of light reaching the image sensor, and so control a picture's lightness or darkness. The shutter speed controls the length of time the image sensor is exposed to light and the aperture controls the brightness of that light.

The Shutter

Shutter speeds are specified in seconds or fractions of seconds. Traditional shutter speeds (see illustration left) are arranged so each is half as fast as the next fastest and twice as fast as the next slowest in the sequence. Changing from one speed to the next is a change of one stop and, depending on which direction you change it, doubles or halves the light reaching the image sensor.

The shutter speed is the most important control you have over how motion is captured in a photograph. The longer the shutter is open, the more likely you are to get blur from subject or camera movement. In studio photography, where the subject rarely moves and the camera is mounted on a tripod, this usually isn't an issue so you can use fast or slow shutter speeds. The only reason not to use very slow shutter speeds is that the camera may automatically increase the ISO setting. If it does (and sometimes even if it doesn't), your image will have more noise than it otherwise would and this reduces image quality. Shutter speeds typically range from about 1/2000 to as long as 8 seconds or so, and a few cameras offer a bulb setting. Bulb keeps the shutter open as long as you hold down the shutter button.

The Aperture

Aperture settings, called f-stops, indicate the size of the aperture opening inside the lens. Traditional apertures (see illustration left) let in half as much light as the next larger opening and twice as much light as the next smaller opening. As the f-stop number gets larger (f/8 to f/11, for example), the aperture size gets smaller. This may be easier to remember if you think of the f-number as a fraction: 1/11 is less than 1/8, just as the size of the f/11 lens opening is smaller that the size of the f/8 opening.

The aperture controls depth of field-the area in a setup from foreground to background that will be sharp in a photograph (page 10). The smaller the aperture you use, the greater the area of a setup that will be sharp. For some pictures you may want a smaller aperture for maximum depth of field so that everything is sharp. In others you may use a larger aperture to decrease the depth of field so that some details are sharp against a softer background.

How wide you can open the aperture depends on the len's maximum aperture its widest opening. For example, a lens with a maximum aperture of f/1.8 opens wider than a lens with a maximum aperture of f/2.6. Larger apertures are better when photographing in dim light and also let you get shallower depth of field. With most zoom lenses the maximum aperture changes as you zoom the lens. It will be larger when zoomed out to a wide angle, and smaller when zoomed in to enlarge a subject.

One desirable feature of a lens is a small aperture. The smallest aperture on some digital cameras is f/8 or f/11. On 35mm film cameras, it may be f/16 or even f/32. Smaller apertures let you get more depth of field and also extend the range of your lighting. If you can't use a small enough aperture, some powerful studio lights will cause your images to be overexposed and too light. Your only control then is to move the lights farther away from the setup, something that may be difficult to do in a small room and which also changes the quality of the light.

Using the Shutter and Aperture Together

You, or the camera's autoexposure system, can pair a fast shutter speed (to let in light for a short time) with a wide aperture (to let in bright light) or a slow shutter speed (long time) with a small aperture (dim light). Speaking of exposure only, it doesn't make any difference which of the combinations is used. But in other ways, it does make a difference, and it is just this difference that gives you some creative opportunities. You're always balancing camera or subject movement against depth of field. This is because a change in one causes a change in the other. Let's see why.

Each exposure setting is 1 "stop" from the next and lets in half or twice the light of the next setting. (Many digital cameras actually let you adjust settings in one-third stops for finer control). A shutter speed of 1/60 lets in half the light that 1/30 does, and twice the light of 1/125. An aperture of f/8 lets in half the light that f/5.6 does, and twice the light of f/11. If you make the shutter speed 1 stop slower (letting in 1 stop more light), and an aperture 1 stop smaller (letting in 1 stop less light), the same amount of light enters the camera so the exposure doesn't change. However, the smaller aperture increases the depth of field slightly and the slower shutter speed increases the possibility of blur.

In the studio, where neither the subject or camera is likely to move, we generally think mainly of the aperture setting because of its effect on depth of field. In aperture-priority mode, as you change apertures the camera automatically changes shutter speeds to keep the exposure constant. All you have to do with the shutter speed is keep your eye on it. If the aperture you select results in a shutter speed below 1/15 second or so, you might turn on noise reduction if your camera has that feature and it doesn't come on automatically.

Exposure Modes

To creatively use shutter speeds and apertures, you have to change the camera's exposure mode. Let's take a look at the modes you usually have access to.

• Automatic or program mode sets the shutter speed and aperture without your intervention. This mode allows you to shoot without paying attention to settings so you can concentrate on composition and focus. For many studio setups, this mode works very well.
• Scene mode has settings designed for specific situations such as landscapes, sports, and macro photography. In desktop photography, the only one of these settings you will be interested in is the macro setting.
• Aperture-priority mode is the most useful mode in studio photography. It lets you select the aperture (lens opening) you need to obtain the depth of field you want and the exposure system automatically sets the shutter speed to give you a good exposure. You select this mode whenever depth of field is most important, as it often is in desktop photography. To be sure everything is sharp, select a small aperture. To throw the background or some other part of the setup out of focus, select a large aperture.
• Shutter-priority mode lets you choose the shutter speed you need to freeze or deliberately blur camera or subject movement and the camera automatically sets the aperture to give you a good exposure. You select this mode when the portrayal of motion is most important. In digital desktop photography where the camera is mounted on a tripod and the subject usually isn't moving, this mode isn't of much importance.
• Manual mode lets you select both the shutter speed and the aperture. You normally use this mode only when the other modes or exposure compensation can't give you the results you want-as when you are using strobes. However, this mode has one big advantage over the other modes. As you move or rotate a subject, the camera's exposure doesn't change as the reflectivity of the subject changes. This means background exposure will remain constant through a series of pictures. This is very important when doing object photography or photographing any series where images will be shown together.