A Short Course Book
Curtin's Guide to Digital Cameras
And Other Photographic Equipment

Exposure Controls

Click to explore exposure compensation.
The universally recognized icon for exposure compensation.
Click to explore exposure lock.
A common icon for AE Lock buttons.
Click to explore autoexposure bracketing.
The standard icon for auto exposure bracketing.
Underexposing by 2 stops kept the background dark while correctly exposing the spotlit subjects.
When a scene is lighter or darker than middle gray you need to change the exposure to capture it the way it looks or it will be too light or dark. The reason for this is because your camera's metering system tries to make every scene have an average of middle gray in the photo.

On the top row are three cards that you photograph with each filling the viewfinder at the time you take the picture. The middle row shows how the camera's exposure system makes all three cards appear gray in the photographs. Only the middle gray card in the center is exposed correctly. The bottom row shows how increasing the exposure for the white card and decreasing it for the black card captures them as they really appear. Only the middle gray card in the center doesn't need the exposure adjusted manually.
Most digital cameras provide ways to override the automatic exposure system to get the exposure you want. The choices to look for are exposure compensation, exposure lock, and autoexposure bracketing.

Exposure Compensation

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. It's easy to use exposure compensation because you can preview your changes on the monitor and reshoot if necessary. Use + exposure compensation when the subject is bright and when it's dark.

Autoexposure Lock (Ae Lock)

You can adjust exposures with a procedure called autoexposure lock (AE Lock) that works much like focus lock. You point the camera so the part of the scene you want to base the exposure on is metered (spot metering works best) and press the shutter button halfway down to set and lock both exposure and focus. While continuing to hold down the shutter button to keep them locked, you recompose and shoot the picture using the locked in settings. This is generally called exposure lock. Some cameras also have a separate AE Lock function that lets you lock exposure independently of focus. The only real difference is that you lock exposure by pressing an AE Lock button instead of the shutter button and it remains locked until you take the picture. Focus is then determined when you take the picture, even if you have changed your position. This is an ideal setting when taking a series of photos to be stitched together into a panorama because all of the photos have the same exposure.

Autoexposure Bracketing (AEB)

When you want to be absolutely certain you have the best exposure, autoexposure bracketing (AEB) mode takes a series of photos— each at a slightly different exposure. It's basically an automated form of exposure compensation but can also be used in conjunction with exposure compensation to shift the sequence up or down the exposure scale. Some cameras let you specify both the number of exposures, usually 3 or 5 of them, and the change in exposure between each shot. Some cameras take all of the pictures with a single press of the shutter button. With others you have to press it once for each picture.

Gray Cards

Since the exposure system is designed to set the exposure to capture a middle gray scene, you can get perfect exposures by using a gray card. When you fill the viewfinder with a gray card and press the shutter button halfway down, your camera will indicate the best exposure regardless of how light or dark the scene is.

Exposure Increements

Many digital cameras let you select 1/3 or 1/2 stop increments for exposure settings. Setting it to 1/3rd stops gives you finer control over the exposure.
When you fill the screen with a gray card and press the shutter button halfway down, your camera will indicate the best exposure regardless of how light or dark the scene is.

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