Exposure Compensation

A better camera won't instantly make your pictures better, but better knowledge of your camera will. One of the most basic aspects of photography is exposure, its principles are universal but is implementation is not. This is why you must know your essential photographic tool, your camera.

Exposure means getting the right amount of light to make a discernible picture. With digital cameras, there are more parameters to instantly control exposure than ever before. The basic parameters are aperture, shutter speed, ISO sensitivity and flash output. These parameters always exist but they are not always under user-control. Some cameras even have additional parameters which influence, to a lesser extend, proper exposure. Almost all modern digital cameras have a fully automatic mode which controls all exposure parameters based on the camera's metering mode. Cameras commonly have several other modes, each of which controls a subset of exposure parameters. A full manual mode lets the user select each exposure parameter.

Exposure compensation is a common feature used to offset a camera's metering. Exposure compensation is made by selecting an offset (measured in EV or stops) which the camera adds (or subtracts) to its metering calculation before selecting the exposure parameters. An EV or stop is a relative measure of light. Each full stop represents twice the amount of light as the previous full stop. For example, EV+1 means twice more light than EV+0 which means twice more light than EV-1. This implies that EV+2 is 4 times more light than EV+0.

In auto mode, the camera controls all exposure parameters, so exposure compensation can be achieved by changing any number of them. Suppose that normal metering without exposure compensation gives a shutter speed of 1/250s , an f-stop of F5.6, an ISO setting of 200 and half-power flash output. Entering an exposure compensation of 2 EV would then change at least one of these parameters. For example, the shutter speed could be set to 1/60s or the ISO could be set to 800. Alternately, the shutter-speed could be set to 1/125 and the flash output set to full-power. Which possibility will occur depends on the camera. This information is generally not available in the camera manual but has an influence on the look of your pictures. In this case, to know your camera, will require experimentation.

Remember that any combination of parameters which gives the same exposure does not necessarily give the same picture! The resulting pictures has roughly the same brightness level, but looks different. Shutter speed changes affect the appearance of objects in motion. Aperture changes affect depth-of-field. ISO sensitivity changes affect image noise. Flash output affects objects that are closer more than ones that are far. Finally, the flash itself also affects the balance and direction of light in a scene.

Greater photographic control is achieved in semiautomatic modes such as shutter-priority mode. In this mode, the user selects the shutter-speed and therefore exposure compensation will affect other parameters. Actual affected parameters will depend on the particular camera. In aperture-priority mode, the aperture is selected by the user and other parameters are set by the digital camera.

Other parameters don't usually have their own mode, but can be selected independently to be automatic or fixed at some value. ISO typically follows this pattern, except some digital SLR cameras do not have an Auto mode. Depending on the camera, automatic ISO can have more possibilities than fixed ISO values. On the Konica-Minolta DImage A2 for example, ISO 64, 100, 200, 400 or 800 can be selected but Auto ISO can select between ISO 64 and 200, including intermediate values such as ISO 160. The same is true for shutter speed on the Minolta DImage 7i.

Flash is usually an unexpected exposure parameter. At the very least, flash can be ON or OFF. An Auto Flash mode will select between ON and OFF but will greatly affect the resulting photograph. Forced OFF mode is easy to understand, it removes flash out of the equation. However, forced ON mode uses the flash but can vary it output intensity. Some cameras have non-exclusive modes which control how flash output is determined such as ADI, pre-fash, manual power, fill-flash and standard flash. On some cameras when the flash is used as the main light source, exposure compensation will only affect the flash output which means that only objects within the flash range will appear brighter or darker.

The bottom line is that exposure compensation is different from camera to camera. Some information may be available in a camera manual, but other information must be discovered through experimentation, The more information you know about your camera, the better you can control the pictures you take.

Full-Manual Mode

In most cases, exposure compensation is disabled (or has no effect) in manual mode, since the camera does not control exposure at all in this mode. It is very common for an exposure compensation button to serve another purpose in full manual mode to avoid cluttering the camera. It is important to note that automatic parameters are misleading in full manual mode. For example, when ISO sensitivity is set to Auto in full manual mode, most cameras keep their ISO sensitivity fixed at the lowest standard sensitivity. The information relevant to your camera should be clearly stated in the camera's manual. This is another case where its important to know your camera.