Overview Picture for Exposure, Exposure Control, Exposure Metering
The exposure metering measures the brightness of your subject. There are different methods of exposure metering that can return different results for the same subject.
However they all return a quantity of light that should be used for the picture. This automatically determined quantity of light can be increased or decreased by an additional manual exposure compensation.
The exposure itself and the brightness of your picture is determined by three key parameters:
- exposure time = the time during which light falls onto the camera’s image sensor
- aperture (or f-stop) = the size of the opening in the lens through which the light falls onto the image sensor
- ISO sensitivity = the image sensor’s sensitivity to the incoming light
There are many combinations of these three parameters that result in the same exposure – i.e. pictures with the same brightness.
If you shorten e.g. the exposure time but widen the opening through which light comes the photo can get the same brightness as before.
Selecting a specific combination of exposure time, aperture and ISO sensitivity is the role of exposure control.
Once exposure is defined each camera allows to „freeze“ the value with the auto exposure lock and to continue using them unchanged even if you point your camera to somewhere else and change the incoming light for exposure metering. This allows you to measure the exposure with a different frame than your later image and to use the same exposure over several shots.
Exposure metering is described in a separate primer, all other elements are subject of this article.