Modern Exposure Metering using multiple Segments
This is called „matrix metering“ at Nikon cameras, „evaluation metering“ at Canon and other manufactures can have other names.
It is the method that manufacturers always set as a default and works as follows:
- The brightness of your subject is measured at many places across the whole or almost the whole frame. This is indicated at the top by coloring the whole frame slightly red.
Modern cameras can use up to more than 1000 different measuring points.
- A complex program logic determines an exposure from all these values, as well balanced as possible.
The logic that is used is a lot more than a simple averaging and the manufacturers are somehow reserved with publishing how their method works in detail.
Important rules that the cameras can use are:
- The brightness of a white area in direct sunlight is the same allover the world. Even more brightness appears only because of reflections or if the sun itself is in the picture.
Exposure metering can recognize by this absolute brightness that an area has to appear white and it can ignore reflections that would bias the exposure.
- The distribution of differences in brightness can be analyzed to keep over- and underexposed areas as small as possible.
- From the patterns of how the brightness are distributed the subject can be assigned to lighting conditions and shooting situation, e.g. bright daylight, overcast day, night shot etc.
With these classifications the program logic can conclude to put a focus more on the brighter or darker areas of the subject.
The methods can get as refined and complex as you can imagine due to the abilities of modern microelectronics, e.g. exposure can be based on faces in thepicture if the camera recognizes faces in the frame before taking a picture. And it can also use color- and information from the autofocus.