Metering: How to get Accurate Exposure
--Dustin Fife
Goal: To have approximately perfect exposure on every shot.
What is Metering: Metering means to measure the amount of light reflecting off your subjects.
How does metering work? The camera has an internal light reader (Through The Lens--TTL) that measures the amount of light reflecting off of the surface of whatever is in your view. When you depress the shutter release button (the button that takes the pictures) half-way, the camera will adjust its settings automatically (depending on what mode you set the camera to) so that the camera can expose the picture how it thinks is best. 
Metering modes:
Typically there are 3 modes:
1. Spot metering: Think of the cross-hairs on the scope of a gun; you tell the gun exactly where you want the bullet to hit, and (if you are accurate) it will only hit in that area. Likewise, with spot-metering, you specify a small part of the scene that you want the picture to take a light reading from. See top-right picture.
2. Evaluative: This is more like using a scope to shoot a bazooka. Instead of blowing up a
very small point on your target, it blows up everything in your “scene.” Likewise, in evaluative metering, it measures the amount of light in your entire scene and comes up with an average exposure. See bottom-right picture.
3. Centered-weighted average: I don’t have any cool analogies for this one. Basically this reads from the entire scene, but it gives more weight to the center of the scene.
Which mode is best? Spot Metering…without a doubt. Though other people have different opinions, I am right. Why am I right? Let’s illustrate an example. Look at the picture on the left. The background is really bright (so bright, it shows up as white) while the little girl is in shadow. What the camera would do with the evaluative and centered-weighted average is take that background lighting into account and make the entire picture too dark. But, with spot metering, you are basically saying to the camera, “I don’t care about anything else, I just want the bride’s face to be exposed properly.”
How do you do it?
Set your camera to spot metering mode. (Note: I have intentionally left out instructions on how to change to spot metering. This is because every brand is different. I would check out your instruction manual to figure out how). Hold your shutter button half-way down until your camera is ready to shoot (ie, has focused and taken a light reading. Typically the camera will beep when it is ready). When the camera lets you know it is ready, fire away and enjoy great exposure.
Some things to be cautious about:
When you take a light reading, the camera will read everything as if it was perfectly neutral--not too bright, not too dark. Well, what happens if you focus on something dark? You got it...the camera will say, "my goodness that's dark. I had better brighten up the exposure." The result: a photo that is "over-exposed" (too bright). And what about metering of off something that is bright? The camera will say, (unfortunately not loud enough) "Whoah....way too bright. Let's crank down the exposure a bit." And the result will of course be a photo that is too dark.
