You shouldnt, however, always leave the exposure to the automatic system. Automatic exposure works well in most, but not all, lighting conditions. At times the lighting can fool any automatic exposure system into producing an underexposed (too dark) or overexposed (too light) image. Although you can make adjustments to a poorly exposed image in a photo-editing program, youve lost image information in the shadows or highlights that cant be recovered. You will find it better in some situations to override the automatic exposure system at the time you take the picture.
Typical situations in which you might want to override automatic exposure include interesting and unusual lighting situation. For example, if you want to photograph into the sun, record a colorful sunset, show the brilliance of a snow-covered landscape, or convey the dark moodiness of a forest, you will probably need to adjust exposure, rather than let the camera make exposure settings automatically.
All light meters, including the one built into your digital camera, operate on the same general principles. A light-sensitive photocell regulates the amount of electricity flowing in the metering system. As the intensity of the light reflected from the subject changes, the amount of electricity flowing through the photocells circuits changes and is used by the autoexposure system to calculate and set the shutter speed and aperture.
Your cameras meter measures light reflecting from the part of the scene shown in the viewfinder or on the LCD panel. The coverage of the meter (the amount of the scene that it includes in its reading) changes just as your viewfinder image changes, when you change your distance relative to the scene or when you zoom the lens. Suppose you move close or zoom in and see in your viewfinder only a detail in the scene, one that is darker or lighter than other objects nearby: the suggested aperture and shutter speed settings will be different than if you meter the scene overall from farther away.
Meter averaging and middle gray
Your exposure meter doesnt "see" a scene the same way you see it. Its view is much like yours would be if you were looking through a piece of frosted glass.
Every scene you photograph is something like a checker board (below), but even more complex. Portions of it are pure black, pure white, and every possible tone in between. Regardless of the elements making up the scene, you cameras meter can average and measure brightness only.
The exposure meter and exposure control system in an automatic camera cant think. They do exactly what they are designed to do and they are designed to do only one thing. Regardless of the scene, its subject matter, color, brightness, or composition, the meter measures the overall amount of light reflecting from the scene. Since the light meter measures only brightness (how light or dark the scene is) and not color, the automatic exposure system then calculates and sets the aperture and the shutter speed to render this level of light as "middle gray" in the photograph. Most of the time this works very well because most scenes have an overall reflectance that average out to middle gray. But some scenes and situations dont average out to middle gray and thats when autoexposure will lead you astray.
A continuous spectrum of tones, ranging from pure black at one end to pure white at the other is contained in most scenes. In simple terms, this continuous scale can be thought of as dividing into a series of individual tones called a gray scale. Each of the tones in this scale has received 1 stop more exposure than the next darkest tone in the series, and one stop less exposure than the next lightest tone. The tone in the middle is called middle gray. A subject uniformly of this tone reflects exactly 18% of the light falling on it.
When you photograph a subject with an overall tone of middle gray, your cameras autoexposure system will set an exposure so that the subject will appear in the final image as middle gray. When you photograph subjects that have an overall tone lighter or darker than middle gray they will also be middle gray in the final image and therefore look too light or dark. As a result, if you photograph first a white card, then a gray card, and third a black card, and each completely fills the viewfinder frame when the exposure is calculated, each of the cards will be middle gray in the captured image.
To make scenes that dont average out to middle gray appear in an image the way they appear in real life, you have to use exposure compensation or some other form of exposure control to lighten or darken the picture.
Types of metering
All parts of a scene are usually not equally important when determining the best exposure to use. In a landscape, for instance, the exposure of the foreground is usually more important than the exposure of the sky. For this reason some cameras offer more than one metering method. The choices might include the following:
Meter weighting can cause a few problems. For instance, a dark object located off center against a very light background may not be exposed properly because it is not located in the area the meter is emphasizing. Or, in some cases, holding the camera vertically may give undue emphasis to one side of the scene. These occasions are uncommon, but when they occur you can ensure accurate readings and exposure settings by metering the subject from close-up. The camera settings can then be overridden if necessary to produce a well-exposed photograph.
When you take a photograph, the exposure isnt uniformly distributed over the sensors surfaceunless you are photographing a subject that is absolutely uniform in tone. Highlights (brighter areas) in the scene reflect the most light, and the areas of the sensor onto which they are focused are exposed a great deal. Darker areas, like shadows, reflect much less light, so the areas of the sensor onto which they are focused receive much less exposure. The perfect exposure retains details in both the highlights and shadows. For the autoexposure system, this is as difficult as your parking a very large car in a very small garage. If there is even a little too much exposure, the image is too light and details are lost in the highlights. If there is too little exposure the image is too dark and details are lost in the shadows.
One way to ensure you get the best exposure is to take three pictures. The first would be at the recommended setting. The second would be lighter and the third darker than the original one. This process is referred to as bracketing because youre bracketing the suggested exposure. You can do this using exposure compensation.
Most scenes that you photograph have an overall brightness of middle gray. Some areas of the scene may reflect 90% of the light and other parts may reflect 5%, but overall the average amount of light reflecting from the scene is 18%, the amount reflected by a middle gray subject.
Whenever you photograph a normal scene with this average brightness, your automatic exposure system exposes it correctly. Typical middle gray scenes include the following:
Lets take a look at some of the most common situations where your automatic exposure system will have problems. Its in these situations where youll need to override the suggested exposure settings.
Scenes lighter than middle gray
Scenes lighter than middle gray, such as beach scenes, or bright sand or snow covered landscapes, reflect more than 18% of the light falling on them. The autoexposure system doesnt know the scene should look bright so it calculates an exposure that produces an image that is too dark. To lighten the image so it matches the original scene, you must override the cameras automatic exposure system to add exposure.
Scenes darker than middle gray
Scenes that are darker than middle gray, such as deep shadows, dark foliage, and black cloth, reflect less than 18% of the light falling on them. Although such scenes are not as common as scenes lighter than middle gray, you will come across them occasionally. If you photograph such scenes using automatic exposure, they will appear too light. The meter cannot tell if the scene is dark or just an ordinary scene with less light falling on it. In either case it increases the exposure to make the scene lighter. When it does this, it overexposes the image and makes it too light. To produce a picture with an overall tone darker than middle gray, you need to override the autoexposure system to decrease the exposure to make it darker.
Subject against very light background
Subjects against a very light background such as a portrait against a bright sky or light sand or snow, can confuse an automatic exposure system, particularly if the subject occupies a relatively small part of the scene. The brightness of the background is so predominant that the automatic exposure system reduces the exposure to render the overall brightness as a middle gray. The result is an underexposed and too-dark main subject.
Subject against very dark background
When a small light subject appears against a large dark background, your autoexposure system assumes the overall tone to be darker than it actually is, because so much of the scene is dark compared to the smaller brighter main subject. The autoexposure system increases the exposure to produce a middle tone. The result is an overexposed and too light main subject.
Scenes with high contrast
Many scenes, especially those with brightly lit highlights and deep shadows, have a brightness range that cannot be completely recorded on an image sensor. When confronted with such scenes, you have to decide whether the highlight or shadow area is most important, then set the exposure so that area is shown accurately in the final picture. In high contrast situations such as these, move close enough so the most important area fills the viewfinder frame. Use exposure lock from that position to lock in the exposure. Another way to deal with high contrast is to lighten the shadows by adding fill flash. A portrait, for example, lit from the back or side is often more effective and interesting than one lit from the front. But when the light on the scene is contrasty, too much of the persons face may be in overly dark shadow. In this case use fill flash or a white reflector card to fill and lighten the shadows.
Hard to meter scenes
Occasionally its not convenient or even possible to meter a scene. Neon street signs, spot lit circus acts, fireworks, moonlit scenes, and many similar situations are all difficult and sometimes impossible to meter. In these cases, its easiest simply to experiment, using the exposure compensation control on your camera. After taking a picture at the suggested exposure, you use exposure compensation to take other exposures both lighter and darker than the suggested settings.
Most digital cameras provide one or more ways to override the automatic exposure system to get the exposure you want.
Exposure compensation lets you lighten or darken the photograph that the camera would produce if operated automatically. To lighten a picture, you increase the exposure; to darken one, you decrease the exposure. The amount you increase or decrease the exposure is specified in "stops." For example, to increase the exposure 1 stop, you specify +1 to open the aperture or slow down the shutter speed. Its easy to use exposure compensation because you can preview your changes on the LCD monitor.
To make changes to exposure, you use exposure compensation control where you can often increase or decrease exposure by two stops in one-third stop increments. Here are some typical settings where youd make these changes.
Just as you can point the camera at an object and press the shutter-release button halfway down to lock in focus, so you can with exposure. For example, with a gray barn sitting in a white snow-covered field, you can use spot metering or move closer to meter just the barn and hold down the shutter-release button to lock in that reading. You can continue holding the button half way down and recompose the picture using the locked in exposure and focus setting.
Some cameras let you adjust the contrast and brightness in your photographs; also referred to as adjusting the tone curve. To visualize the effects these controls can have, adjust the brightness and contrast settings on your TV or computer monitor. Some cameras allow you to adjust brightness and contrast at the time you take a picture. With others, you can only do it later using a photo-editing program.
Brightness raises or lowers the brightness of the entire scene to make everything lighter or darker.
Contrast adjusts the differences between the brightest and darkest areas in the image. You can increase or decrease the contrast.