A camera’s built in light meter wants to even out the scene, but it does so by treating everything as a midtone. It aims to bring the exposure as close to 18% midtone grey as possible. While with certain scenes this produce beautiful exposures, if you were photographing a snowy scene, the snow would appear grey. If, on the other hand, you were photographing a naturally dark scene, such as woodlands, the image would appear too light.
For exactly this reason, the exposure compensation function exists. It can be accessed from the function menu, or on the camera body. It is shown as +/-. 0 is regarded as the ‘correct’ exposure, but if you were to shoot a snowy scene, you would require positive exposure compensation, (ie. +1 or +2), and if you were to shoot a forest you would need negative exposure compensation.
If you want to, you can let the camera do the work, but help it out a little by selecting a metering mode suitable for your shooting conditions. Cameras have several metering modes that you can select as shown below.
Multi-zone, or Evaluative
This is the default metering mode for most cameras. It divides the frame into sections, and takes a reading from each of these sections, and so is considered good for landscape photography. Difficult lighting conditions, and overcast days are challenging for this mode, and so it is not always suitable.
Centre Weighed metering
This mode takes a reading from the entire frame, but focuses more on the central 60-80%. It give fairly predictable results, and is less likely to be fooled by bright or dark spots at the edge of the frame. It can be fooled by bright skies or foregrounds, which are often typical of landscapes.
Potentially the most accurate, this mode takes only 1-5% of the frame into account, but does a good job. It will still try to expose for a midtone. Agood example of a midtone is the blue sky on a summers day or a sunlit lawn. If the area the spot cover is darker, then you will need to ue negative compensation. If, on the other hand, the spot is brighter, apply positive compensation.