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The exposure lock feature in most digital cameras is borrowed from 35mm camera technology, and it is one of the handiest tricks you can learn and master. Exposure lock is almost always achieved by applying slight pressure to the shutter release—not enough to activate the shutter and take the picture, but enough that you feel the button move and the camera itself respond.
Here’s what happens when you take a picture:

1. Apply slight pressure to the shutter release button.
2. As you feel it depress slightly, the camera’s autofocus lens locks the current subject into sharp focus.
3. At the same time, the camera’s exposure meter measures the light and locks in an exposure.
4. Apply more pressure to the shutter release to press it in all the way. The camera then takes the picture and saves it to memory.

Cameras that use a fixed focus lens often do not include an exposure lock feature, or they use a separate button to lock the exposure. Inexpensive “budget” cameras may fall into this category. Refer to your camera’s manual to see if it has exposure lock, and how to use it.
The magic of exposure lock is that as long as you continue applying light pressure to the shutter release, the camera will use that “locked-in” exposure information regardless of where you later point the camera. You can lock in exposure information for the sky and then point the camera at your feet and snap the shutter release all the way. You’ll take a picture of your feet using the sky’s exposure data. You probably wouldn’t want to do that since the result will be totally underexposed, but it gives you an idea of the potential. Exposure lock is a great tool for telling the camera that you’d like to take a picture with the exposure data from one specific part of the scene. Imagine, for instance, a scene like the one in the Color Insert. Here we have a boy at the beach, standing in the water at sunset. The image can be dramatic, but only if exposed properly. We’d like to capture the overall dark tones inherent in a sunset scene, with exciting splashes of color to light up the subject in a subtle way. Just pointing the camera at the scene might result in the camera averaging the bright and dark bits of the picture, generating an image that might as well have been captured at midday. That would be quite ordinary and not at all what we want.
Instead of taking the average picture, here’s what you should do, step by step:

1. Frame the scene in your viewfinder so you know what you want to photograph.
2. Before actually taking the picture, point the camera up into the sky. Include the brightest part of the sky that doesn’t also include the sun—that might be overkill. (Sounds like guesswork? It is, a little. This is art, not science. You can take the picture, see if you like the result, and reshoot as necessary.)
3. Press the shutter release partway to lock in the exposure information. You should sense that the camera has also locked the focus at the same time.
4. Recompose your picture. When you’re happy with the scene in the viewfinder, press the shutter release all the way to take the picture.

Obviously, you could also choose the spot meter (if your camera has one) and lock the exposure with that instead of the default center-weighted or matrix meter method in step 2. It’s up to you.
A center-weighted camera would probably average the light in the darker surrounding areas and determine that it needed to select a fairly wide-open aperture to add light to the scene. But that would wash out the poor little wolf. Instead, the best solution is to select the spot meter, frame the wolf carefully, and press the shutter release partially to lock in exposure on the bright subject. Then reframe the picture and shoot. As you become more confident with your ability to visualize compositions and exposures, you can try different things to get the desired effect.

As I mentioned earlier, it’s really the camera’s exposure sensor—known as an exposure meter—that does the majority of the work when figuring out how to shoot your picture. It decides how much light is needed to adequately expose your picture. So it should come as no surprise to learn that cameras distinguish themselves by the kind of meter they use. Some meters are better than others at metering a scene and applying the right exposure.

Center-Weighted Meters
In the old days, most cameras came with a simple center-weighted light meter. This meter measures the light throughout the image, but applies more weight, or importance, to the central part of the scene in the viewfinder. The assumption—usually a good one—is that you are most interested in the stuff in the front of the picture, so the camera tries to get that part of the scene exposed properly.

Matrix Meters
A new innovation in photography is a kind of sensor that has been around in 35mm SLR photography for a decade or more. Called matrix metering, this system is most prevalent in Nikon models, all the way from the CoolPix 950 and CoolPix 995 to the high-end D1x. The matrix meter, is famous for its ability to properly expose tricky scenes by balancing the lighting needs of five or more discrete regions within a picture. Instead of concentrating primarily on the middle, matrix meters gauge the light in many parts of the scene at once. If your camera has a matrix meter mode, you should use it most of the time.

Spot Meters
The last major kind of light meter is called a spot. The spot meter is never the only kind of meter in a camera; instead, it’s an option that you can switch to if the center-weighted or matrix meter fails you.
So when should you use the spot meter? Any time you are trying to photograph a scene in which a small subject must be exposed properly for the picture to work—and its lighting is different enough from the rest of the scene that you’re worried it won’t come out right otherwise. Imagine, for instance, that you are trying to photograph someone who is standing in front of a brightly lit window. If you let the camera decide the exposure, the bright light from the window will radically underexpose the subject. So switch on the spot meter and expose the picture based on the subject. Yes, the window light will be overexposed, but that’s okay—the important part of the picture is the person.
Varying the metering mode—especially the spot meter—is best used in conjunction with the third technique, exposure lock.

Most digital cameras come equipped with an exposure compensation control, usually referred to as the EV adjustment. The EV control allows you to lock in and use the camera’s recommended automatic exposure setting, but then adjust that value up or down based on factors that you’re aware of but the camera may not be smart enough to see. Each Exposure Value (EV) corresponds to changing the exposure by one stop, such as going from 1/60 to 1/30 (this is a change of +1 EV since it doubles the exposure) or 1/15 to 1/30 (this is –1 EV since it reduces the exposure by half). Take When the camera is set to overexpose the scene by one stop or EV, however, the scene is much better exposed. To use the EV control on your camera, follow these steps:

1. Size up the scene you want to shoot. Decide if it calls for over- or underexposure.
2. If you need to add light to a scene to properly expose it, add exposure by setting the EV control to +1. If you instead need to underexpose the scene, set the EV control to –1. Here’s a typical digicam display set to EV +2:
3. Take the picture and review the picture in the LCD display. If you don’t like the result, adjust the EV and shoot it again.

Most cameras allow you to adjust exposure by up to three EVs, either positive (overexposed) or negative (underexposed), and some models also allow you to set the EV in increments of one half or one third of an EV at a time for more fine control over your scene.
You have a digital camera at your disposal, so frame your picture and take the shot. If you don’t like the results, take it again with different settings!

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.