Automatic Exposure (AE)

Some photographers purposely overexpose an image to create a unique look, such as shown here. Stuart Dee / Getty Images

Automatic exposure (AE), sometimes shortened to auto exposure, is an automated digital camera system that sets the aperture and/or shutter speed, based on the external lighting conditions for the photo. The camera measures the light in the frame and then automatically locks in the camera's settings to ensure a proper exposure.

Having a proper exposure is very important, as a photograph where the camera doesn't measure the light properly will end up overexposed (too much light in the photo) or underexposed (too little light).

With an overexposed photo, you can end up losing details in the scene, as you'll have bright white spots in the image. With an underexposed photo, the scene will be too dark to pick out details, leaving an undesirable result.

Automatic Exposure Explained

With most digital cameras, you don't really have to do anything special or change any specific settings to have the camera make use of automatic exposure. When shooting in fully automatic modes, the camera adjusts all of the settings on its own, meaning the photographer has no control.

If you want a little bit of manual control, most cameras give you a few limited control options, yet the camera can continue to use automatic exposure. Photographers usually can select one of three different shooting modes with limited manual control while maintaining AE:

 

  • Aperture priority allows the photographer to set the aperture value, and the digital camera then automatically determines the shutter speed to create the proper automatic exposure.

     

  • Shutter priority allows the photographer to set the shutter speed, and the digital camera then automatically determines the aperture to create the proper automatic exposure.

     

  • Program mode allows the photographer to make changes to either the shutter speed, the aperture, or both, and the digital camera then automatically determines the ISO to create the proper automatic exposure.

    Of course, you also can specially control the exposure for the scene by shooting in full manual control mode. In this mode, the camera doesn't make any adjustments to the settings. Instead, it relies on the photographer to make all of the adjustments manually, and these settings end up determining the exposure levels for a particular scene, as each of the settings works in tandem.

    Making Use of Automatic Exposure

    Most cameras will set the automatic exposure based on the lighting in the center of the scene. 

    However, you can use an non-centered composition and lock in the AE by centering the object that you want properly exposed. Then either hold down the shutter button halfway or press the AE-L (AE-Lock) button. Recompose the scene and then press the shutter button fully.

    Adjusting the AE Manually

    If you don't want to rely on the camera to set the exposure automatically, or if you're shooting a scene with particularly tricky lighting conditions where the camera can't quite seem to lock in on the proper settings to create the proper exposure, you have the option of adjusting the camera's AE.

    Most cameras offer an EV (exposure valuation) setting, where you can adjust the exposure. On some advanced cameras, the EV setting is a separate button or dial.

    With some beginner level cameras, you may have to work through the camera's on-screen menus to adjust the EV setting.

    Set the EV to a negative number to reduce the amount of light reaching the image sensor, which is useful when the camera is creating overexposed photos using AE. And setting the EV to a positive number increases the amount of light reaching the image sensor, used when the AE is underexposing photos.

    Having the proper automatic exposure is a key to creating the best possible photo, so pay attention to this setting. Most of the time, the camera's AE does a good job of recording an image with the proper lighting.

    On those occasions where the AE struggles, though, don't be afraid to make adjustments to the EV setting as necessary!