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Not all lighting situations are easy to shoot; that’s why photography is both an art and a science. Specifically, it’s fairly easy for real-life scenes to trick your camera’s exposure sensor and consequently under- or overexpose a picture. You can fix that tendency to some degree on the PC afterward, but it’s much better to expose the picture correctly to begin with. That’s because an over- or underexposed image is missing information about colors, texture, and detail that can never be restored afterward; only the moment of exposure can ensure that all the information will be in your image. What are some examples of tricky photos? There are many, but just a few problems tend to surface most frequently. Take a look at the trio of images in the Color Insert to see what happens if you base your exposure on the wrong part of the picture. These kinds of problems have several solutions, and you can experiment to see which works best for you in various situations. Here are some ways you can correct your exposures when you see a problem in the viewfinder:

• Use exposure compensation Use the EV control on your camera to intentionally under- or overexpose your pictures beyond what the camera’s exposure sensor recommends.
• Switch metering modes You can use a different kind of exposure meter to account for very “contrasty” images.
• Use exposure lock Lock your exposure on a different part of the image, and then recompose the picture and shoot.

To see how to use each of these exposure techniques, keep reading.

Instead of relying on an all-automatic or all-manual exposure system, you can compromise and use your camera’s shutter or aperture bias, if it has one. The idea with these controls is that you select either an aperture or a shutter speed, and the camera automatically selects the other half of the exposure for you.
These settings are usually used to find the right balance between freezing (or blurring) motion in a picture and focusing attention on the subject by sharpening (or blurring) the background of an image.

Keep in mind that these recommendations are just a starting point. Here are some tips that can help you zero in on your ideal exposure:

• Your camera will recommend an ideal exposure, which you can accept or reject. If the camera considers your setting out of bounds, it may flash a warning in your viewfinder.
• Adjust your exposure based on the ISO setting. If your camera is set to an equivalent of ISO 200, for instance, the Sunny 16 Rule would call for a shutter speed of f/16 and a shutter speed of 1/250.
• Make adjustments for brighter or darker scenes. If you’re shooting in a dark room, for instance, the Sunny 16 Rule doesn’t really apply—but it’s a good starting point. Open the aperture or lengthen the shutter speed to account for the reduced light.

At an ISO of 100, which many digital cameras use for general-purpose photography, you might want to rely on the traditional Sunny 16 Rule for a starting point. The Sunny 16 Rule is very old—it dates back to the earliest days of analog photography—and it suggests that when shooting outdoors in bright sunlight, choose an aperture of f/16 and a shutter speed that’s equivalent to your film speed. Since few cameras offer the capability to choose 1/100, most photographers that rely on this rule use 1/60 or 1/125 when shooting with ISO 100 film. The following table identifies other acceptable combinations. (All these add up to the same overall exposure.)

Shutter    SpeedAperture
1/1000     F/4
1/500       F/5.6
1/250       F/8
1/125       F/11
1/60         F/16
1/30         F/22

The most basic manual exposure control you can exert over your camera involves setting both the aperture and shutter speed. Some digital cameras allow you to set these controls as if you had a fully automatic 35mm SLR. There are two kinds of cameras you may run into with this capability:

• Point and shoot Most point and shoot digicams that include manual settings for aperture and shutter speed require you to use the LCD display to make onscreen menu changes. The Epson PhotoPC 3000Z, for instance, uses a pair of buttons on the right side of the LCD display to adjust shutter speed, and another set of buttons on the bottom of the screen to change the aperture settings. It isn’t hard to do, but you need to remember first to set the camera to its Manual Exposure mode, then remember which buttons do what for fine-tuning exposure.
• SLRs A few digital cameras—pro-style SLRs, for the most part—use traditional SLR controls for making manual adjustments to exposure. Specifically, you can turn the aperture ring on the lens to change the f/stop or use a control on the camera to accomplish the same thing, as you can see in the following illustration. Shutter speed is likewise affected with a dial on the camera body, and you can look through the viewfinder to keep tabs on the setting.

To set exposure manually, you must choose a shutter speed and aperture combination that will properly expose your scene at a given ISO. This is a great exercise for new photographers who are serious about learning photography theory.

Though I talk quite a lot about shutter speed in this blog, the reality is that few digital cameras have a real shutter in the sense that 35mm cameras have shutters. The 35mm cameras usually have a physical barrier that blocks light from entering the chamber where the film is stored. This mechanism—the shutter blade—moves lightning fast, able to deliver shutter speeds as fast as 1/8000 of a second. That’s fast. Digital cameras, in comparison, often don’t have real, physical shutters. You can verify this yourself with a simple experiment: When you press the shutter release on a 35mm camera, you can hear the quick, metallic click of the shutter blade opening and closing. A digital camera may not make any noise at all. Or it might make an obviously fake “click” sound through the camera’s speaker (Kodak and Sony cameras tend do this). It can, in fact, be quite difficult to discern whether a picture has been taken at all until you get used to the way digital cameras work. When I had my first digital camera, I actually had to look at the LCD display on the back of the camera to see if the picture was captured, or if for some mysterious reason the camera was still waiting to grab the shot. So if there’s no shutter blade, how is the picture actually taken? Often, the CCD is simply turned on long enough to expose the picture. Since the CCD is an electronic component that acts as the camera’s film, it can be actuated electronically for whatever exposure time is needed. In addition, the camera’s aperture may close completely to keep light from reaching the CCD—prolonged exposure to sunlight can damage this sensitive part of the camera. But the aperture needn’t spring open and closed as quickly as the shutter blade in a 35mm camera, so the sound it makes isn’t as dramatic. You’ll hardly notice it at all.

Here’s what usually happens: when you apply pressure to the camera’s shutter release, the camera’s microcomputer samples the scene in front of the lens and determines how much light is needed to adequately expose the scene. With most digicams, the camera selects a shutter speed and aperture combination that is sufficient to get the job done. But, you might be wondering, how does it choose? After all, there are a lot of shutter speed/aperture pairs that will work. To take the same properly exposed picture at ISO 100, any of these combinations should be exactly the same:
Usually, the camera uses the following logic:
The photographer wants to take a picture using the fastest available shutter speed to minimize camera shake and motion blur from objects moving inside the picture.
Though there are some exceptions, most cameras tend to choose the combination that allows for the highest available shutter speed, limited only by how small they can make the aperture given the current lighting conditions and ISO setting.
This isn’t always what you want your camera to do, though, and in fact you might sometimes want to choose a slower shutter speed, overexpose the image, underexpose it, or perhaps base the exposure on a completely different part of the picture. That’s why you might want to investigate your camera and look for controls that let you tweak the shutter speed and aperture.

If you find yourself in a situation in which the lighting isn’t quite right for your picture, it’s time to bump up the camera’s ISO value. Remember that not all cameras come with ISO adjustments, so review your user manual to see if this applies to your particular model. You’ll probably find it in the onscreen menu system, displayed in the LCD screen on the back of your camera.
Here are some situations in which you might need to increase the ISO:

• You’re shooting in a low-light situation, such as early evening or indoors Naturallight photos have a certain appeal, and by increasing the light sensitivity of your camera you may be able to shoot a picture without using the flash at all. Using natural light can eliminate harsh shadows and produce more natural colors.
• Your subject is too far away for the flash to have any effect During the day you might be outdoors and want to take a picture of something, but there’s not quite enough light—such as in winter or during very overcast conditions. Your camera wants to use a flash, but your subject is just too far away. The flash on your digital camera has a very limited range. So to properly expose your picture, you need to use “faster film”—that is, increase the camera’s ISO setting.
• You’re shooting at night Most digital cameras have very little ability to take pictures at night or in near total darkness. A few models can extend the shutter to about half of a second, but that’s about it. As a result, if you want to capture anything at all with a night shot, you’ll need to increase the camera’s light sensitivity to maximum.

If night photography interests you, investigate what I refer to as “performance” digital cameras—cameras that include manually adjustable shutter speeds and apertures. Cameras like the Nikon CoolPix 995, the Olympus C-4040, and the Canon PowerShot G2 are the sorts of models that are better equipped to deal with very low light such as you’ll encounter at night. Using more full-featured cameras let you perform long exposures for light trails, glowing illuminated signage, and other special effects.

You can use the ISO control built into your digital camera to vary its sensitivity to light and thus mimic the effect of using different grades of 35mm film. This can come in handy in a number of situations, such as when you’re shooting in particularly high or low light levels. Remember a few key facts about your camera’s ISO ratings, though:

• It’s not a real ISO adjustment, in the sense that your camera doesn’t have real film. Each camera maker has a somewhat different way of implementing this feature, but they all use ISO numbers since that’s comparable to 35mm film, which most people are at least a little familiar with.
• Don’t leave your camera set at the highest ISO all the time. Some folks think that by setting their cameras to the highest sensitivity, they’ll be prepared for anything and won’t have to muck with the camera menu when they’re on the go, trying to take pictures. In reality, boosting your CCD’s sensitivity to light also increases the amount of digital noise you’re capturing. More ISO means more fringing, artifacts, and digital detritus. Sometimes that’s unavoidable, but stick with the lowest ISO value you can get away with most of the time.

All that talk about f/stops, shutter speed, and ISO settings may seem irrelevant to your digital camera, but it’s not—all cameras use these concepts, even though they’re sometimes disguised fairly well. The main difference between a digital camera and a chemical film camera, of course, is the fact that digicams don’t use film. That means you never load anything that has a specific ISO value into the camera. So how does the camera actually work? Simple. When light enters the camera at the moment of exposure, it doesn’t hit light-sensitive silver halides that are fixed in a chemical broth. Instead, the light hits a computer chip called a charge coupled device (CCD). The CCD is light sensitive, and each of its many pixels register changes in light just like the film’s many grains of silver react individually to light. In other words, the silver grains in film and the pixels in a CCD are essentially the same thing. They contribute to your picture in the same way, and both are the smallest components that make up your picture.
The CCD makes a picture by noting the variation in light rays that travel through the camera lens. The CCDs pass this information on to the camera’s microprocessor in the form of varying electrical charges. The image is transformed into digital bits and stored on a memory card. Your camera’s CCD functions like the film in a 35mm camera, except that it differs in one important way—you can’t swap the CCD out of your camera and insert one with more light sensitivity for low-light photography. The CCD is a permanent part of the camera. Camera makers understand that you might need to change the camera’s light sensitivity on occasion, though, and that’s why many cameras can have their ISO rating “adjusted” on the fly, whenever you want. In essence, what this does is allow you to “turn up” or “turn down” your camera’s sensitivity to light.