Chances are the light meter you use is built into your camera.
Chances are also that you have no real access to it. Since most people use mobile phones for taking pictures, the companies don't consider it necessary, but it is.
Just as there are apps that will allow you to control ISO, Aperture and Shutter-Speed on a mobile device, there are things we can buy to turn them into a light meter.
This graphic represents a typical light-meter display in an SLR camera. (Some film cameras use a sweeping needle instead of lighted bars)
This meter uses individual bars, each one illuminated based on the amount of light being detected in that particular region.
If the only bar illuminated is in the middle ("0"), then according to the meter, we have a balanced exposure and that's a good thing.
If the lighted bars push more into the positive end of the meter, I am over-exposed. Meaning the exposure has too much light.
Lighted bars in the negative territory, indicate I am under-exposed. I have too little light.
Whether automatic or fully-manual, everything depends on getting an accurate light-meter reading of our subject.
Failure to do so means our exposure equation (ISO+Shutter-Speed+Aperture) will be wrong right from the very beginning and our photographs don't come out.
So it's important to understand just how cameras see and record the world, sometimes the light-meter is wrong and we need to know what to do.
There are two kinds light-meters: Reflected and Incident
Reflected light-meters are the ones built into a camera.
As the name implies, it measures the light being reflected off our subjects, everything from a flower to a distant mountain and beyond.
This type of meter evaluates the world not in color, but in shades of gray. It searches for the halfway point between full-black and full-white (something called 18% gray), then sets the exposure based on that.
The downside to reflected metering are very bright scenes like the sun, snow, a sandy beach, a lamp or any large bright surface.
Such meters are fooled by this brightness and will adjust the exposure downwards to compensate. Our image is now under-exposed, with nice white snow being rendered as a shade of gray.
If we're photographing a large black surface, such as a monument, this will not be represented accurately, either. The exposure will be shifted upwards to a dark shade of gray, over-exposing our image.
Here's reflected metering at work...
For one example, I pointed and pressed the shutter button just like everyone else would. The street lights are very bright and since my camera is pointed in that direction, they are a big part of the metering equation.
The result is an under-exposed image.
Now, if I pointed the camera down and and away from the lights, so I am only metering on the pumpkins and the people, I get a much better picture.
I set the camera accordingly or use the exposure-lock feature on my camera (most have them), then reposition my scene before taking the picture.
It's the same thing if shooting outside in bright daylight.
Pointing the camera in the direction of the sun, makes it part of the metering equation and the exposure will be thrown off. But by turning away from the sun and metering, the exposure will be accurate.
A small adjustment may be needed here and there, but we will most certainly be in the ballpark.
This is why people starting out in photography are advised to shoot in manual. We are making adjustments as we go and before long, all these elements will become second nature to us.
An incident light-meter is a hand-held device.
This kind of meter measures the amount of light falling on our subject. We can be photographing something right in front of us or a mountain miles away.
Incident meters do not factor reflecting light into the equation. Snow is rendered as being white, not gray. The same with a black monument.
The downside to an incident meter happens when shooting outdoors in constantly changing and may require regular metering.
Some incident meters have the ability to also read reflected light, making them more versatile. Plug-in accessories are available to turn our mobile device into incident meters.
COMPARING THE TWO
Here's our Frosty the Snowman blow mold peaking out from under the snow.
Each photo shows a different metering type and is straight from the camera, no corrections other than crop and alignment.
For reflected-metering, I shot in full-automatic and just accepted the camera default settings. For incident-metering, I was in manual-mode and set the camera to whatever my hand-held meter indicated.
Hands down, the incident-meter was the most accurate of them all. As far as reflected metering goes, the closest was Spot.
SLR cameras provide multiple metering options to choose from. This way, we can direct the meter to specific areas for more precise measurements.
Spot is a small dot, dead-center in the viewfinder. An example for using spot is a bright sunny day when our subject is in the shade. Spot narrows our metering to a very small area and disregards everything else.
Center-Weighted is a slightly bigger area than Spot and for decades, was the only one available to film camera users. Like with Spot, this meters a limited area.
Matrix or Multi-Zoned is for an entire scene. It attempts to balance the light from many different areas, including Spot, and actually does very well most of the time. This is where a company's advertising departments boast of the attributes of their camera's metering system.