Calibration to „Medium Gray“
The first principle needed to understand exposure metering is:
All exposure meters are calibrated to render a subject of low contrast as a medium gray.
Here an example for illustration: A white and a black piece of paper are put onto the floor. The two marbles in the middle are to distinguish the two and to give the camera something to focus on.
In the following table are pictures where only the white or only the black paper fills the frame completely. And each of the two papers is there three times using different types of exposure metering.
|exposure metering||white paper||black paper|
(using multiple segments for metering)
In case you wonder that the pictures look all the same: Yes, they are all taken without any manipulation and exposure compensation. After taking the overview picture I have only moved the camera to above the papers and changed the type of exposure compensation.
About the comparison of different types of exposure metering:
The methods differ basically in two points – at which places of the image they measure the brightness of your subject and how the different results are put together to some kind of average. One of the following pages explains this in more detail, here this high level principle is enough.
If the brightness is the same everywhere the different methods must all end up with the same result.
The modern „3D color matrix metering II“ of a Nikon D7000 with 2016 RGB sensors shows – like any other modern camera would do – the same results as the center-weighted metering that was the standard of the 1980s when there was no digital photography yet.
About the comparison of the white and black paper:
The camera electronics can measure only the quantity of light that is coming in and it knows the quantity of light that the image sensor can process before a photo will appear overexposed – so purely white.
But the camera cannot distinguish like a human being between a brightly lit dark area and a white subject in dim light and therefore exposure metering is calibrated to an assumed medium brightness of your subject.
If your subject meets the assumed average brightness the exposure will fit, as in the overview picture with both the white and the black area in it. If not your picture will get too dark or too bright.
This first principle is applicable to all digital cameras, regardless of their price and it is valid today as it was already for decades of analog photography before digital photography came up.