What is Exposure Compensation in Photography?

Almost every digital camera available in the market come with a wide array of automatic shooting modes. Herein the camera decides the exposure settings on behalf of the user. However, even though cameras are programmed to set the perfect exposure, not always will the camera correctly expose the photograph. In photography “exposure” is a term to justify that the photograph is not too bright or too dark. A balanced exposure means the photograph is pleasing to the eyes and subject is perfectly identifiable.

However, the as practically there is unlimited variation of light patterns both naturally and artificially, it is impossible for a camera to perfectly expose every time. So, the camera manufacturers to avoid the dissatisfaction of users makes an inbuilt exposure correction tool available. It gives the photographer a good amount of control by alleviating miscalculations done by the camera.

The exposure correction tool is Exposure Compensation – which gives an easy way to correct improper exposures done by the camera. There is a sliding scale in found on most digital cameras. Usually it ranges from -3.0 on the left to +3.0 to the right. There will be an indicator on the scale which shows the amount of exposure compensation made. If the indicator is set to zero, there camera will make an exposure as perceived by metering without taking any input from you. If you find the exposure made by the camera is dark, you can override the settings by adjusting the compensation level towards plus. Whereas if the camera made a brighter exposure, you have to dial in a minus compensation. In both ways, you are telling the camera that you are not completely satisfied with the settings made by the camera and you are adjusting it slightly.

How Exposure Compensation works?

In the first example we try to photograph a girl in front of a blackboard. In this scenario the camera sees all those dark colours and will try to properly expose by overexposing, but it will make the girl to be overexposed as well as seen on the resulting photograph on the right.

In the second example a boy is in front of a whiteboard. Since there is a lot of white, it forces the camera to properly expose the scene by underexposing the scene, but it will make the boy to be underexposed as well as seen in the resulting photograph on the right.

In both the above examples the camera tried to adjust the scene for a perfect exposure but the resultant images were either overexposed or underexposed due to the presence of the darker or lighter background. Here, comes the utility of the Exposure Compensation tool. If you see the photograph is overexposed you need to dial in the negative numbers to set the right exposure or the positive numbers if the photograph is underexposed. The Exposure compensation tool gives you the power to override the exposure interpreted by the camera.

Why cannot a Camera always perfectly expose?

A camera’s exposure meter is always calibrated at Middle Gray level, which means the gray color that is in the middle between complete white and complete black and reflects 18% of the incoming light. See image below:

When you point your camera at your subject and take a meter reading, the meter will consider the intensity of light being read as middle gray. This creates exposure issues in the resultant photographs.

As in the first example if a scene has dominating black subject (the blackboard) the camera exposure meter will consider this black subject as middle grey and hence the subject will be over exposed as seen on the right of the illustration. Therefore to overcome this problem, we need to decrease the exposure so that black subjects look black.

A similar miscalculation occurred in the second example where there is a dominating white subject (the whiteboard) and the exposure meter treats white as gray in the image and hence the subject was underexposed as seen on the right of the illustration. Here to overcome this problem, we need to increase the exposure so that the white subject look white.

Exposure Compensation and Semi-Automatic Camera Modes

Exposure Compensation is a way to alter any exposure while shooting in Program, Aperture Priority or Shutter Priority modes.

In Program mode the camera can adjust the aperture or shutter speed in order to accommodate the adjustments you make in Exposure Compensation. Let us say a your camera has metered a scene at f/8.0 and 1/500s, then if you make an adjustment of +1 EV on the Exposure Compensation dial, it may result in a setting of f/8 and 1/250s or f/7.1 and 1/500s. By dropping the shutter speed to 1/250s, the camera doubled the amount of light reaching the sensor (because the exposure lasts for twice as long). Likewise, by opening the aperture to f/7.1, the camera doubled the amount of light reaching the sensor (because the aperture was opened a full stop).

In Aperture Priority mode, the Aperture is controlled by you. Accordingly the camera will adjust the Shutter Speed based on the Exposure Compensation. For the above example, where the camera metered the scene at f/8.0 and 1/500s, with +1 EV, the camera will change the shutter speed to 1/250s.

In Shutter Priority mode, the Shutter Speed is controlled by you. The camera will adjust the Aperture based on the Exposure compensation. Using the same example, where the camera metered the scene at f/8.0 and 1/500s, with +1 EV the aperture will be set to f/7.1.

It may be noted that in both Aperture Priority and Shutter Priority modes, the camera may also adjust ISO if you have the ISO set to automatic mode.