In very basic terms:
Metering means to ensure that u properly expose ur scene. There are 3 kinds of mode (a) matrix metering (b) centre weighted (c) spot metering
(a) matrix metering - normally used in landscape photography to properly expose the whole scene, the camera will determine the light across the whole picture and try to find the best setting to properly expose everything.
(b) centre weighted - normally used in portrait setting or other scenes where there is a main object that occupies abt 70% of the picture center. The camera will try to properly expose the 70% of that picture and "ignore" the background. So for example, if ur object is black and ur background is white, the camera will meter the black object, and will try to adjust itself to bring out the black object's details. In order to do so, it got to expose more. So wat u get is a properly exposed black object against a over exposed background. Remember the background is already white, and because of the need to expose more to bring out the black details, the white background will be overexposed.
So i think u get wat i mean, the camera will only care abt the center 70%.
(c) spot metering - similar to the centre weighted metering, but just that in spot metering, we meter only a small spot eg. a 6mm diameter circle in the picture. This is used for example, when there is a small frog in a bright picture. if u use matrix metering, the camera will evaluate the whole scene and think that the whole picture is too bright, and hence under expose the picture to generate a proper exposure for the whole scene. But in doing so, the small frog may be further underexposed and become dark frog.
in addition, if u use centre weighted metering, remember it assigns abt 70% to the center of the picture, but the frog is only very very small in the picture! hence the end result will also be the camera feels that the scene is too bright and underexpose.
Thus in spot metering, u aim the center crosshair on the frog to meter the frog. This will ensure that the frog is properly exposed and be seen! but of course the background may not be properly exposed.
In apeture priority, shutter priority and program mode, u need to select ur desired metering style: (a) matrix metering (b) centre weighted (c) spot metering and then the camera will take care of the metering, thus u just concentrate on selecting the correct apeture, shutter speed or iso value/white balance to shoot ur photo.
But for manual mode:
1) Switch ur camera to manual mode
2) Select ur desired metering style: (a) matrix metering (b) centre weighted (c) spot metering
3) if using spot metering or centre weighted metering, remember to point ur centre crosshair onto ur desired object.
4) look into ur viewfinder and u will see a horizontal bar something like this + lllllll|llllll -
5) what u got to do now, is to turn ur apeture dial or ur shutter dial to make sure that the horizontal bar is centered
6) half press ur shutter release so that the exposure is locked i.e. even if u point at other places in ur scene, the metering wont change due to metering other objects.
7) frame ur photo nicely and shoot.
Hope u understand all that i just typed..have fun
