In a scene that is unevenly illuminated, the object may appear against a bright background or a light source may exist behind the object. In this case, to the camera the incoming light is bright, so the camera will reduce its exposure (for example, by increasing the shutter speed). However, this reduces the brightness of the overall scene, making the object in the image too dark.
The backlight compensation (BLC) function provided on many cameras helps to mitigate this problem. The image scene is divided into six or seven (with two repeated) areas. The camera measures the light level of each area and calculates exposure based on their weights. For example, the central area may be assigned 9 times more weight than any other area. If the exposure is calculated mainly based on the light level in the central area, an object in the central area can always be clearly imaged.
In scenarios with a very bright background, BLC can provide an ideal exposure for the foreground object, regardless of the position of the object in the scene. In some cases, BLC may cause background overexposure or image noise. Figure 1 shows the imaging of a scene before and after BLC is enabled.