Tips for Adjusting Autoexposure Results on Your dSLR - dummies

Tips for Adjusting Autoexposure Results on Your dSLR

By Julie Adair King

Autoexposure is a useful tool in digital photography, and for most subjects and lighting conditions, produces good results. But it’s not foolproof, and it can’t read your mind on occasions when you want to purposely under- or overexpose an image to evoke a certain photographic look or mood. Not to worry: Even using autoexposure mode, you may be able to adjust the brightness of your next shot by using the features outlined next: exposure compensation and autoexposure lock (AE Lock).

Another possible autoexposure fix lies in the exposure metering mode. Again, that setting tells the camera which part of the frame to analyze when setting exposure. So if the metering mode is set to measure the entire frame, you may get different exposure results if you shift to spot metering mode, which enables you to base exposure primarily on your subject. Even then, though, the picture may still come out too bright or too dark for your tastes, so you may need to combine metering mode adjustment with these other autoexposure fixes.

Applying exposure compensation

This feature, often referred to as EV (exposure value) compensation, bumps the exposure up or down a few notches from what the camera’s autoexposure computer thinks is appropriate.

The setting is often marked with a little plus/minus symbol like the one shown in the margin, and you typically choose from settings such as +0.7, +0.3, 0.0, –0.3, –0.7, and so on, with 0.0 representing the default autoexposure setting. (If you’re an old-school photographer, it may help you to know that these settings represent exposure stops. For example, EV –0.3 reduces the exposure by one-third of a stop.)

How you adjust the amount of compensation varies from camera to camera, but the settings themselves are simple to understand:

  • For a brighter exposure, raise the EV value. The scene below is a classic example of the problem that occurs when you have a dark subject against a bright background: The camera, choosing exposure settings that average out the brightness values of the entire frame, left the palm tree too dark. So an EV compensation of +1.0 was applied to produce the brighter result on the right.
brighter exposure
The original autoexposure setting left the palm tree too dark; raising the EV setting to +1.0 produced the brighter result.
  • For a darker exposure, lower the value. In this image, the original exposure didn’t have the drama that was the focus with this composition, and highlights in the sunlit brick and the candle flame are blown out: Areas that should span a range of brightness values have been so overexposed that you’re left with a blob of white pixels. To get the more artistic result on the right, EV –1.0 was used.
Here, the autoexposure system blew out the highlights; lowering the EV value solved the problem.

On some cameras, the exposure meter that helps you assess exposure in manual exposure mode appears whenever you enable exposure compensation. For example, if you dial in an adjustment of EV +1.0, you see a bar under the 1.0 mark on the positive side of the meter. This readout can get confusing because you naturally assume that you’re about to overexpose the picture.

But remember: The meter is based on what the camera considers the ideal exposure — which is what you get when the EV value is 0.0 (no compensation). When you set the EV compensation level to +1.0, you are, in essence, asking the camera to overexpose the shot by one stop over what it thinks is ideal.

Make it a practice to reset the Exposure Compensation value to EV 0.0 after you finish shooting the subject that required the adjustment. You can easily forget that you enabled the option and then not be able to figure out why everything is too bright or too dark when you move on to your next subject. If you do get weird exposure results on a shot, check this setting first to make sure that you didn’t leave compensation turned on by accident.

AE Lock (autoexposure lock)

When you use autoexposure, most cameras continually measure and adjust exposure settings from the time you press the shutter button halfway to set focus until the time you take the picture. Usually, that system works well because it accounts for any lighting changes that may occur at the last second. But on occasion, you may want to interrupt the continuous exposure adjustment and lock in the current settings. Some cameras offer an AE Lock feature that does just that.

Your camera may have a button specifically set aside for this function, or you may be able to assign it to a Function (FN) button or another button. Many cameras offer a button labeled AE-L/AF-L, which locks both autoexposure and autofocus, respectively, when pressed. (You often can customize the button so that it locks exposure only if you prefer.)