Spot metering takes only, you guessed it, a spot of the picture and sets the exposure for that spot. To use this mode change your camera to spot metering. Select the focus point you want to use and aim it at what you want exposed correctly and the camera will evaluate the exposure for that section of the picture. Generally speaking you are going to want to use matrix metering for most of the pictures you take. However spot metering is is useful for tricky lighting situations.
I have been thinking about writing this post for a while now but needed some examples. So, while I was in Disney over the holidays I ran across all kinds of tricky lighting situations and got some great examples. The pictures themselves are b-sides at best but they are good examples of the different types of metering. Spot metering will change your cameras shutter speed or aperture or both depending on what shooting mode you are in. So, I have included the shutter speed and aperture of all the pictures so you can see the difference.
In all of the examples below I have the matrix metering shot first and the spot metering shot second. This first shot is pretty terrible but it is a good example. As you know I love blue skies and this sky is far from blue so I switched to spot metering and exposed for the hotel. Again, neither of these are great pictures but it shows the extremes at which it can change your picture.
|f 5.0 1/100 Auto White Balance ISO 200|
|f 7.1 1/200 Auto White Balance ISO 200|
|f 2.5 1/25 Auto White Balance ISO 200|
|f 4.0 1/60 Auto White Balance ISO 200|
|f 11.0 1/400 Auto White Balance ISO 200|
|f 8.0 1/250 Auto White Balance ISO 200|
|f 5.6 1/125 Auto White Balance ISO 200|
|f 4.5 1/80 Auto White Balance ISO 200|
|f 11.0 1/500 Auto White Balance ISO 200|
|f 14.0 1/800 Auto White Balance ISO 200|