Fully automatic, often called Program mode, it determines both the shutter speed and the aperture that will produce an accurate exposure. Aperture priority, as the name implies, allows the photographer to input an aperture (ƒ-stop) and the camera automatically determines the corresponding shutter speed. Shutter priority does the reverse, allowing the photographer to dial in shutter speed so the camera chooses the aperture.
In all of these automatic exposure modes, the camera is making a reading of the light it sees through the lens, often factoring in subject distance as well as the colors in the scene, and comparing it to a database of “similar” images in order to help it determine the most appropriate exposure for the scene it’s facing. Still, even with all of this, as well as highly refined metering modes that value certain portions of the frame over others, cameras still get fooled on a regular basis and deliver exposures that are too light or too dark, instead of “just right.”
There are three common scenarios in which camera meters are fooled and produce incorrect automatic exposures. These are scenes that are either very dark or very bright (a black cat in a black hat in a dark room, for instance, or a white cat in a white hat in a snowstorm) or particularly contrasty scenes, especially those containing backlight. An indoor portrait subject standing against an open window, for instance, is strongly backlit. The light source behind the subject makes the scene appear bright, yet the subject’s face as presented to the camera is in shadow—effectively a silhouette. This, in particular, is a recipe for a significant underexposure—as in the example shown here. In this type of lighting scenario, exposure compensation excels.
The exposure compensation camera setting is fairly easy to understand. It’s a scale, from neutral (0) to plus and minus a few stops, usually in one-third-stop increments. On the Fujifilm X-Pro2 pictured here, and with many other cameras, the exposure compensation dial is atop the camera and can simply be rotated to +2 or -3 or whatever amount you choose in order to apply the corresponding compensation to the exposure to account for conditions that are likely to produce an errant ready and an under or overexposure.
It helps to think of the camera as wanting to make everything appear middle gray. If a scene is much darker in reality, expect the camera to make it appear middle gray (i.e. too light). And if a scene is much lighter, the camera will want to make it too dark in its efforts to achieve middle gray. It’s not quite this simple given how intelligent camera meters have become, but middle gray is still at the heart of the exposure compensation issue.
In a particularly dark scene, because the camera is likely to compensate for all that darkness and produce an automatic exposure that is much too bright, dialing the exposure compensation setting to -2 will tell the camera to underexpose by two stops in order to compensate for the naturally dark tones in the scene.
Likewise, in a backlighting scenario, the camera might underexpose by two or three stops based on the brightness of the background and the subject in shadow. Turning the exposure compensation dial to +2 tells the camera to compensate by overexposing two stops from what it believes to be the correct exposure.
Exposure compensation can also be quite useful in more typical exposure scenarios for a more subtle exposure control. For instance, if you simply want to brighten up a scene, add +2/3 stops of light by turning the exposure compensation accordingly. Even +1/3 can be helpful if you prefer a slightly brighter-than-average exposure.
In any lighting scenario and no matter what you shoot, exposure compensation is a quick-and-easy way to fine-tune automatic exposures and help the camera render the image more to your liking.