Metering is something most of us probably take for granted in modern cameras but it is essential for every single shot you take, whether you're shooting in full Auto mode or completely manual.
The metering system measures the light entering the camera - through the lens (TTL) in the case of digital SLRs - and it then translates this information into relative shutter values, aperture values or stops. So when exposing manually you can use the digital reading to match what the light meter deems is well exposed; or when shooting in Auto it will provide the camera with the information to select the appropriate shutter speed and aperture values.
When it comes to making a camera, and to testing it, the metering system is as much an integral part as the focusing, processor and sensor. Each camera manufacturer has its own unique metering system and strives to achieve better ways of evaluating the light coming through the lens.
Most cameras will provide you with a choice of exposure modes for your metering as, although the system is clever enough to work out how to expose the image, it cannot predict if you want to get creative with your exposure or bring detail out of certain areas. When there is high contrast in the scene or strong backlighting, the metering system is forced to choose between shadow and highlight areas, as the dynamic range of the sensor cannot record to the same extremes that we can see with the naked eye. By changing the metering mode, you can choose whether you meter universally from the scene, give priority to certain areas, or simply select an area of the scene.
Metering is only a guide for exposure as, in most cases, it will tell you the exposure needed to achieve the most even range of tones. But for creative shots this may not be what you require, and in some cases a balanced range can leave some areas over or under exposed that may be important to your shot. On these occasions it may be possible to adjust your metering mode to one more suitable, or even switch to fully manual and override its suggestions. However, a much quicker method is often to use exposure compensation. This allows you to dial in a positive or negative value in stops, in relation to the given metering. Using the instant results from the LCD screen, it is easy to quickly access your image and make adjustments based on what you see.