We're going to start with the basics of exposure. Your camera is pre-programmed to assume that every photo that you take should be of an "average tone". The word "tone" confuses some people, but it just means a shade of color. So, light blue and light purple would be the same tone as would dark blue and dark purple. Get it?
The reason your camera makes that "average tone" assumption is pretty simple -it isn't smart enough to know what's in front of it. How can a camera tell if it's pointed at a white wall with dim light hitting it or a gray wall with really bright light hitting it? The answer is -it doesn't. It will assume that the wall is an "average tone" and your photo will come out an average gray tone either way.
Once you understand the camera's assumption you can use it to your advantage by anticipating and correcting for the camera's mistakes in judgement about a scene.
Let's pretend that this is the scene in front of you:
This is a pretty typical photo, so the camera's default metering should get it right. But, we're going to walk through exactly how the camera makes the decision of what the level of brightness should be for this scene to get a feel for how it works in more challenging photography situations.
"Evaluative" or "matrix" metering:
When you have your metering set to "scene", "matrix", or "evaluative" mode (they all mean the same thing, but different camera makers use different terms) what the camera does is it takes a look at the entire scene, averages it out, and decides on the correct brightness for the entire photo. For typical photos like the one above this default mode works very well since the majority of the photo is made up of colors of an "average" tone. But if the scene contains a lot of lighter than average or darker than average tones it won't work nearly as well.
Let's take a look at a scene where the camera's assumption about the scene being of "average tone" is wrong:
The background of this photo is white. Not only that, but the background makes up more than 50% of the photo, so that's a lot of white! If the camera assumes that the scene as a whole should be of an "average tone" what you're likely to end up with if you take this shot with the default settings is something like this:
Hopefully that makes sense. ...the camera doesn't know the background is white. All it knows is that the scene should be of an "average tone". So, to compensate for all that bright white, it tones the whole photo down and makes the white look gray.
That's why, when your scene is not of "average tone", you need to choose one of the other metering modes that your camera offers. Your choices of metering modes usually include "spot" and "center weighted".
"Spot metering" is usually my preference since it gives you the most control. The idea is rather simple, you just point your center autofocus point at something in your scene of "average tone" and your camera will only use that exact point to determine your exposure.
So, if in the apple photo above you pointed your center AF point at the apple, the scene would likely come out correctly with the white background appearing white instead of gray as it would if you used "evaluative" or "matrix" metering. If you pointed your center AF point at the white background, you could expect the photo to turn out the same or darker than what you'd get using "evaluative" or "matrix" metering.
Let's try one more. Here's a new scene:
What would you expect to get if you use "evaluative" or "matrix" metering for this scene?
What would you meter off of if using "spot" metering when taking this photo?
...the answers are at the bottom of this article.
"Center-weighted" metering uses a fairly large circle in the middle of your scene to determine exposure while ignoring the tones at the edges of the photo. Since it's not clear what is and is not included in this circle where your exposure is evaluated, this mode doesn't give you the same level of control as "spot" metering does. So, I tend to use "evaluative" when I want to let the camera do the work, or "spot" when I feel I need to take control over the exposure for the shot. "Center-weighted" is kindof an in-between mode that I just don't find all that useful.
So, if you took the photo of the purple flower on the black background in "evaluative" or "matrix" metering mode, you'd expect the camera to try to make all that black more of an "average tone". That means, it'd brighten it up to make it gray instead of black; so, you'd get a washed out version of the photo. If, however, you used "spot" metering and metered off of the open purple flower you'd likely get the black background to come out black as it did in the photo above.
Questions? Leave a comment or ask about it on my "photography_beginners" mailing list!