Autoexposure controls the amount of light that is used to expose the camera’s image sensor. It does this by adjusting the aperture and the shutter speed.

Proper exposure is basically determined by averaging the amount of light needed to produce an image with good highlights, middletones, and shadows.

The camera has a built in light meter that evaluates the total amount of light reflected from the subject. It then averages this light and sets the correct exposure.

Digital cameras can have more than one type of metering system however. So it is important to learn how your particular camera meter measures light as well as any options to change the way it does this.

Averaging Meter System

An averaging type of metering system is very common so we’ll look first at how it works and how to use it correctly.

With an averaging type system the meter reads the entire scene then calculates the exposure based on the assumption that there are a few highlights, a few shadows and a full range of mid-tones.

Very often this type of metering is center weighted, which means it gives more emphasis to the center or foreground area of the image than to the other areas. If your main subject is in this part of your photo, you should get a properly exposed image.

	In the image to the left you see an example of an ‘average type scene’. Shot against a grey background the subject has a good balance of light, middle, and dark areas. But if you have lots of bright area in the scene, such as snow or a bright sky, this method may not fare so well. A lot of black area will also cause the meter to render an incorrect exposure.
The same subject, this time with a bright white background on the left and a black background on the right. In each case the auto exposure was fooled.

The bright white caused the meter to calulate more total light, so the exposure was reduced. Now the background is grey and the shadows lack detail. The overall effect is a muddy looking image.

With the shot on the right the excess black caused overexposure. Notice how washed out the tones are and the lack of detail in the whites. All 3 shots were done using autoexposure.

You can also demonstrate this averaging effect another way. Get a white, a grey and a black subject. The example below shows each item properly exposed.

Now shoot each subject separately, filling the entire frame for each shot. The subjects will all become a similar grey tone, as shown in the shots below.

This is the way a camera sees things, and it is different than the way our eyes see. And by understanding this difference, you begin to see as the camera does and develop your eye and brain as a photographer. Learning to see as a photographer is an important step to improving your photos.

Using Exposure Lock

Ok…so how do we control this type of metering to get the results we want? Most digital cameras come with exposure lock. This allows you to lock the exposure by pressing the shutter release button halfway down and holding it there while you recompose your picture.

Experiment with this by placing a person in front of a bright background, such as a white wall or bright sky. Try turning the camera verticle, filling the majority of the frame with the subject. Lock in the exposure.

Now turn the camera to a horizontal composition with your subject off center and take the picture. Your subject should be well exposed.

TIP:

One caution here…often the focus and exposure are both locked when the shutter release button is pressed half way, so this technique is limited. As long as the subject is about the same distance from the lens in the final composition as it was when you used the lock function then you will be OK.

There are many additional controls for exposure and we’ll explore them all in the advanced features pages. If your camera doesn’t have these options, you will still want to know what they are…especially if you want more creative control over your photography. These options can be extremely important to getting the results you want.

Here’s to better photography…

Al Hannigan