In the previous articles, we have reviewed the Exposure Triangle, and how it is affected by Aperture, Shutter Speed and Sensitivity.
In these articles, we mentioned how different combinations of these variables can lead to the same exposure. The question now is: what is the right exposure for a given subject?
Of course, there isn't one answer to this question. However, all cameras in their automatic modes do choose one exposure, so how do they do that?
If we are shooting a plain piece of paper in a controlled uniform lighting, the exposure setting is very simple: it is set in such a way that the piece of paper appears "midtoned".
It gets more complex if different pieces of the scene have different luminosities: white versus dark subjects, variations of lighting, ...
Most cameras will have several exposure metering modes, the most common being spot, center-weighted, average, partial and evaluative.
With spot metering mode, the exposure is set such that one specific spot of the scene (usually the center or the point of focus) appears midtoned;
In partial mode, the exposure is calculated based on a region around the center (or focus point). The exposure is set such that the elements in this region appear in average as midtone;
In average mode, the exposure is calculated based on the average luminosity of the entire scene;
Center-weighted is a combination of partial and average, with a higher importance (usually ~80%) for the cenral (or focus point) region and a lower importance for the edges. This mode allows to take into account the great variations of luminosity in the edges, while still putting more emphasis on the center (or focus point).
The evaluative mode, also called multizone, matrix, segment or honeycomb is in fact a camera-specific mode. It is a complex algorithm where the camera processor measures several parameters of several regions and decises on an exposure setting. The parameters that are taken into account can be, on top of luminosity, the colors/hue, distance to subject, in focus/out of focus, ...