The Metering Scale
Most cameras today use a process known as TTL metering, or, through the lens metering, which means that the camera examines the light coming through the lens and adjusts the settings accordingly.
You can see your camera’s meter readings, by switching to manual mode and looking through the viewfinder. The meter is usually found on the bottom or side and often appears as a number scale with a tiny triangle pointer above the numbers that indicates whether an image is properly exposed. The scale will have a 0 at the center, with numbers on the right to indicate overexposure, and on the left for underexposure. Getting the pointer as close to 0 as possible is usually ideal as this indicates proper exposure.
If you depress the shutter halfway, you will engage the meter and as you move the camera around you’ll notice the meter readings change, depending on where you’re focusing. This is because different objects are lighter or darker.
While the metering scale is great, it really only gives you a partial story. It tells you how much light you are getting, but it doesn’t show you where in the frame it is coming from or how much is coming from where. This is where metering modes come in.