Using Exposure Compensation
The almost universally recognized exposure compensation icon.
When metering an object against a background,the background may have more influence over the exposure reading than the object does.
Many digital cameras display an exposure scale to guide you when you use exposure compensation.
Autoexposure isn't perfect. Because it is designed to accurately capture a scene that has an average reflectance of middle gray, it's puzzled by setups that don't fit this ideal. In those cases you have to override the autoexposure system to get the results you want. Let's begin by looking at some of the most common situations where your automatic exposure system will have problems and then explore how you override the suggested exposure settings.
- Subjects lighter than middle gray, such as a white china plate, reflect more than 18% of the light falling on them. The exposure system doesn't know the scene should look bright so it calculates an exposure that produces a middle gray image that is too dark.
- Subjects that are darker than middle gray such as black cloth, reflect less than 18% of the light falling on them. The exposure system calculates an exposure that makes the image middle gray and too light.
- The contrast or difference in brightness between the subject and the background can fool an exposure system, particularly if the subject occupies a relatively small part of the scene compared to the background. The brightness of the background is so predominant that the automatic exposure
system adjusts the exposure to render the overall brightness as a middle gray. If the main subject is lighter than the background, it will be overexposed and too light. If it's darker than the background, it will be underexposed and too dark.
- Depending on how you arrange the lighting, some subjects may be too contrasty with brightly lit highlights and deep shadows. The range of brightness may exceed the range that can be captured by the camera. In these cases you normally adjust the lights to balance out the light and lower the contrast. However, you can also decide whether the highlight or shadow areas are most important, then set the exposure so those areas are the ones shown accurately in the final picture.
- Occasionally it's not convenient or even possible to accurately meter a subject. Neon lights, lava lamps, and many similar subjects are all difficult and sometimes impossible to meter. In these cases, it's easiest simply to experiment until you get the results you want.
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.
When confronted with any subject lighter or darker than middle gray, you use exposure compensation to lighten or darken the photograph that the camera would otherwise produce. 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. It's easy to use exposure compensation because you can preview your changes on the monitor and then immediately review the results.
- To lighten a picture, you increase the exposure (+). This is useful for setups where the background is much lighter than the subject, or when photographing very light objects, such as white china on a white tablecloth.
- To darken an image, you decrease the exposure (-). This is useful for setups where the background is much darker than the subject, or when photographing very dark objects, such as black china on a black tablecloth.
Top. Here are three cards that you photograph with each filling the viewfinder at the time you take the picture.
Middle. The camera's exposure system makes all three cards appear gray in the photographs. Only the middle card, the middle gray one, photographs correctly.
Bottom. Using exposure compensation to increase the exposure for the white card and decrease it for the black card captures them as they really appear. Only the middle card, the middle gray one, needs no exposure compensation.
One way to ensure you get the best exposure is to take at least three pictures. The first exposure is made at the setting recommended by the camera (0). For the second, exposure is increased (+) to make the image lighter. For the third, exposure is decreased (-) to make it darker. This process is referred to as bracketing because you're surrounding or "bracketing" the suggested exposure. When shooting the series you can select the increment in the exposure between images. In some cases it may be a full stop, in others a half or third stop. In addition, if the lighting is really confusing, you can take 5 or more images at different settings. Some cameras have an autobracketing feature that does this for you automatically and let's you specify both the number of exposures and the change in exposure between each shot. When you later open the images in a photo-editing program, one of them should be the best possible exposure to work from.
Here a collector's knife is photographed on a white background using the recommended setting (left) and then using exposure compensation
(right). Using the recommended setting the green blade has gone black and the background and white
handle have gone gray. Exposure compensation increased exposure to capture the image like it actually looked.
Here the knife was photographed against a
middle gray background and the exposure is perfect.
Here the knife is photographed on a black background using the recommended setting (left) and then using exposure compensation (right). Using the recommended setting the green blade is too light, the background has gone gray, and the white handle is burned out without detail. Exposure compensation decreased exposure to capture the image like it actually looked.
If exposure compensation or
bracketing don't give you the control you need, you can always switch to manual exposure mode.