Two examples of over-exposing:
Evaluative metering for teddy on white linen made the white linen look grey. (see image on the left) Why? Because the camera's metering system is exposing for mid grey. So where a large amount of white dominates the frame, white will try and expose to mid grey. To ensure the white is really white I have over-expose by a full stop. (see image on the right).
Camera metering for this image is not ideal. the dynamic range is outside the camera's capability. I have a range of about 6 stops with my camera. This scene is closer to 10. The amount of very bright sky meant that my camera's metering was going to give me a silhouette. As I wanted detail of both the structure and the men I over-exposed the shot by one and a third stops. It meant the sky would be blown out but it was a bright uninteresting sky that was not going to add anything to the image. Ideally the shot would be better taken in better lighting conditions. Late in the afternoon for example, when the contrast is not so great. The image is really quite flat. However there may be occasions when you don't get another opportunity to take the picture and so you have to work with the existing conditions.
Two examples of under-exposing:
Camera metering exposes for mid-grey. Selecting the camera's average exposure would result in the horses looking grayish. Certainly not the black that they are. I wanted to retain as much detail as possible but make the horses appear black. I therefore calculated that an adjustment of a full stop would give me a nice blackness in the horses. I did lose a small amount of detail which I have highlighted in blue. I found this amount of lose of detail perfectly acceptable. The areas lost had nothing of any importance in them and were dark shadows. Moving up a third of a stop while showing those spots within the camera's range meant I lost depth of colour where I really wanted it.