Understanding the internal light meter of my camera is one of the best skills I've ever learned. It helps me to shoot in fully manual mode, so that I rarely look at the back of my camera. Let me tell you how to get a correctly exposed picture from the first click just by using your camera's internal light meter.
Most of the time I shoot with strobes and then I use a handheld light meter. But when evaluating the ambient exposure, or if I shoot landscapes, I rely on the internal light meter. This saves me time looking at the back of the camera on each click to check exposure, and helps me quickly adjust the basic settings (aperture, shutter speed, ISO) without taking my eye off the viewfinder.
First of all, let's talk shortly about the focus points of your digital camera. These are the points similar to the ones on the above image. You will see them through the viewfinder or in Live View. When you half-press your shutter button, the auto-focus kicks in and judges what focal distance the lens has to use so that the object at this point is in focus. The focus point can be changed from auto to a specific spot. I usually select the center one. The chosen focus point can also be used by the light meter to judge exposure.
What the Light Meter Does
The light meter is an internal function of the camera that gives you a visual indication of how dark or light the image is according to the camera. The light meter is this little thing you see in Live View or through the viewfinder:
Most cameras can change the range of the light meter from -3 to +3. In the example I will use a -2 to +2 indication. When the white cursor is at zero, this means according to camera's internal algorithm, that's the right exposure. A deviation to the left means the image is darker, and if the white cursor is to the right, the image is lighter.
The light meter sees a scene (the picture you point the camera to) as points with different brightness as if the image was all black and white. Each point is evaluated against the camera's zero, which is a 18 percent gray. On the digital monitor that color looks like a 50 percent gray, or the middle shade of gray between black and white:
For each pixel the camera evaluates it checks if its brightness is more or less than this middle gray color. If the majority of pixels are darker than it, the light meter cursor goes to the left. If the majority are lighter, it goes to the right.
As I said above, the camera evaluates pixels from the scene. Depending on your metering mode camera setting, it may check the area of the whole frame or just a fraction of it. There are two basic metering modes that tell the camera how much of the frame to check. Digital cameras offer more, but once you know the principle, you can decide which one you want to use.
Center Weighted Mode
Whatever this is called in different cameras, it is indicated as a blank rectangle. This mode tells the camera to compare all pixels in the frame to the middle gray and calculate the average brightness. If the average brightness is less than the middle gray the cursor goes to the left, otherwise, to the right.
This mode will give you an overall impression if the scene is well exposed. The problem is it won't tell you if there are way overexposed or way underexposed spots in the frame. That is the reason I do not use that mode when I see very bright or very dark spots in the scene.
Partial/Spot Metering Mode
It tells the camera to look at a small area around your current focal point (yes, that's the focus point I told you above). If you place your currently selected focus point to the part of the scene you want to evaluate, the light meter will indicate if it's lighter or darker than the middle gray color. This is the metering mode I usually use because it's more precise.
How I Evaluate My Exposure in Manual Mode
As I said, I use a spot metering mode. When I first look at the scene, I find the lightest and darkest points.
I point my current focal point (usually the center one) to each of those areas and see how much the exposure deviates from the zero according to the light meter. I tend to keep the exposure of the lightest and darkest areas between the -2 and +2, if possible. Although your camera manufacturer may tell you the camera offers 10 or more stops of dynamic range, I try to keep the scene in the range of three to four stops (there are four stops between the -2 and +2 indicator).
One stop means the light amount or brightness is twice more or less. When the indicator is at -1, this means there's twice less light than the middle gray point. Two or minus two shows that the light is four times more or less than the middle gray. When shooting in raw file format you can bring back easily about two stops of information later in post. That's why keeping the image within the -2 and +2 range is relatively safe.
If the lightest or darkest parts of the image are outside that range, I change some of my settings (aperture, shutter speed, ISO) so that it makes the image lighter or darker and I find the compromise set of settings where I'm in the safe range.
I do not try to put my darkest or lightest parts in the zero when metering them. That's why they are the "lightest" and the "darkest" areas of the frame. They will always be above or below the zero point. The goal is to see how light and how dark those areas area so the image is in a reasonable exposure range.
Then I press the shutter, and as I'm in manual mode, I take photographs from that point of view without changing my settings anymore.
Know Your Light Meter
When your light meter tells you something is "-2" and it is not that dark, this means you are underexposed. You have to decrease the aperture number or decrease the number indicated by the shutter speed or increase your ISO to make the image brighter. But if the area you point it to is really dark, a value of "-2" may be just right. The same with areas that are indicated as "+2." If that's an area with a white snow, it's perfectly fine to leave it at "+2." That is the correct exposure of the snow. If you point your light meter at the palm of a hand, it should be usually around the zero mark.
Go out, switch into manual mode, and try reading the light meter indicator. Point your camera to bright and dark scenes and train yourself to understand when your exposure is correct without checking the back of your camera. It is quite easy and you will be surprised how soon you can take correctly exposed pictures from the first click.