When composing for a photo the camera will examine the scene and try to determine the best exposure. Depending on the settings how the camera decides what it needs to correctly expose can vary. There are 3 distinctly different exposure modes and it is important to know which mode to use to achieve the desired effect. There are three metering modes are known as Matrix/Evaluative, Partial/Centre Weighted and Spot metering.
The matrix metering system examines the entire scene and tries to find the best balance of correct exposure from everything it sees. It is a highly sophisticated system that often produces the correct results. When the camera examines the scene the importance of the areas that are correctly exposed carry greater significance closer to the centre and lower in their importance as they move further away to the edge.
Although the camera takes into account all the light in the scene it prioritises the Centre area only. The amount of the scene it takes into account can vary. The default is around 13% of the total area but this can be adjusted. The image below shows the adjustments levels that can be made.
Spot metering will take a light meter reading from a very specific point. This is normally the centre of the scene but it can be changed in the camera settings to take the spot meter read from where the point of focus is.
This is an example of a typical 39 point focus system. the spot metering will take the exposure from where the focus point the user chooses. Spot metering is great for creative images such as taking sunset silhouettes.
This image here was taken using matrix metering. The camera has examined the entire picture and attempted to make an even exposure from all the areas. It does very well and the outside of the window frame, the leaves and the scene beyond are all exposed nicely
This next image uses Centre weighted metering and the camera has attempted to correctly expose the green leaves and the scene beyond it. It has created an even exposure in the areas required and now the wooden window frame and its surround have been ignored. Because the area beyond was much brighter the camera has darkened it to get a correct exposure and compared to the previous matrix metered image the window frame has darkened too.
The final image was taken using spot metering. For this shot I took the read from the statue in the scene beyond the window. The camera has ignored all other areas and took the picture to expose for this area. The result is that the scene beyond the fence looks fine but the leaves of the bushes and the wooden window frame have all become much darker.
There is no absolute right or wrong image here, each works in its own way. It all comes down the photographer and what they wanted from the image.
In an effort to find practical uses for the different metering modes I decided to look for opportunities to use them.
The Matrix metering system will work for most situations but works best when the scene is evenly lit. In this example I took an image around the promotion of the Amazon Kindle. The goal was to combine great literature with the image and I named it Dark Satanic Mills after Blake’s famous poem. It was a cloudless day and though I may have some exposure problems with light reflecting off the giant wind turbines and long dark shadows but with the sun very high it did not cause problems with either so I stuck with Matrix Metering. As you can see that the entire scene is correctly exposed and although the subject is partly in shadow the camera has dealt with this well.
In this example matrix metering would try to correctly expose both the flower and the darker green hedges. Because the hedge is darker it might lead to the flower becoming over-exposed as the camera will lighten the picture. By using a centre weighted exposure the camera will take the reading from only the flower as its priority leaving the main subject correctly exposed. I have placed the subject on the right rule of thirds and in order to both meter and compose the image I had to take the light meter reading first and use the cameras exposure lock to save the reading. I then re-composed and shot the image with the saved metering reading.
Spot metering offers perhaps the greatest levels of creativity. By exposing from a small area in the photo you can get some striking effects. One of the most commonly taken images is the silhouette like the example shown here. For this image the setting sun was overpowering the meters and even centre weighted was trying to find a balance between foreground and background. With the contrast between the two parts of the image so great I took a spot meter reading direct from the sun. the resulting effect was this suburb silhouetting of the wild flowers in the foreground.