Photographic theory and practice- Light meter and metering modes

Light meter and metering modes

To take a photo of a subject exactly as it is, or as you imagine it, accurate measurement of the light on and around the subject is vital. The measurement of this light is called "metering." 
Metering is how your camera determines what correct shutter speed and aperture should be, depending on the amount of light that goes into the camera and the sensitivity of the sensor

Metering options often include partial, evaluative zone or matrix, center-weighted and spot metering.

You can see the camera meter in action when you shoot in manual mode, look inside the viewfinder and you will see bars going left or right, with a zero in the middle

If you point your camera at a very bright area, the bars will go to "+" side, indicating that there is too much light for the current exposure setting.
If you point your camera at a very dark area, the bars will go to the "-" side, indicating that there is not enough light.
You would then need  to increase or decrease your shutter speed to get to "0', which is the optimal exposure, according to your camera meter

Zone System

The Zone System is a technique that was formulated by Ansel Adams and Fred Archer back in the 1930's. It is an approach to standardised way of working that guarantees a correct exposure in every situation, even in the problematic lighting.

Middle Grey

The camera metering is designed to give correct readings under average circumstances. This means that the camera would look at the scene and try to render it as average reflectance (18% reflectance), which is middle grey ( a value between pure white and pure black). When a scene contains too much bright, however, the camera tries to provide it as average so it darkens it causing under-exposure.

In the opposite direction, when a scene contains too much dark, the camera tries to render it as average so it causing over-exposure

reflected Metering, incident metering

Because reflected metering reads the intensity of light reflecting off of the subject, it is easily fooled by variances in tonality, color, contrast, background brightness, surface textures, and shape. What you see is often not at all what you get. Reflected meters do a good job of reading the amount of light bouncing off of a subject.

Because incident metering reads the intensity of light falling on the subject, it provides readings that will create accurate and consistent presentation of the subject’s tonality, color, and contrasts regardless of reflectance, background color, brightness, or subject textures. Subjects that appear lighter than middle grey to your eye will appear lighter in the finished image. Subjects that are darker than middle grey will appear darker.