What is exposure?
Exposure is simply the amount of light capturing on photosensitive material like film or digital sensor.
What is correct exposure?
How much light should hit the sensor for how long?
How does the camera determine the “correct exposure? The camera has a small device inside called exposure meter or light meter that measure the amount of light hitting the sensor. It is the cumulative of light over a duration that results in total amount of light falling on the sensor. The camera compares the result of total amount of light with a standard that is equivalent to 18 percent gray. Brighter than 18% gray, the camera will treat it as over exposed. Darker than 18% gray, the camera will treat it as under exposed.
Here is the example of 18% gray.
Will the camera meter always give us correct exposure all the time?
Well, the answer is no and that is why the decent camera will have those manual buttons for us to override the setting provided by the camera.
The camera exposure metering is based on 18% gray so it provides the setting (aperture and shutter speed combination) to produce 18% gray or the mid-tone photo. Do we want all out photos to look like 18% gray mid-tone? Definitely no. Sometime we want white, sometime we want black.
For the extreme cases below, when we take the photo for the black board or the white board. What do we get straight out from the camera? Gray, 18% gray.
Without doing anything or just using the auto mode from the camera, we are going to get under or over exposure photos under these condition. How do we change it? We can compensate for it with the exposure compensation button +/- of most camera since we know the reason behind the wrong exposure setting of the camera.
For the black board example, our camera thought it is under exposed if it give us black because it wants to give us gray and that result in over exposed photo. What we need to do is simply subtract light by compensate to the – side.
For the white board example, it is the opposite that our camera thought it is over exposed if it give us white because it wants to give us gray and that result in under exposed photo. What we need to do is simply add light by compensate to the + side.
This applies to the scenario of beach, snow, water reflection or white wall background. Our camera meter will think it is very bright and will under expose the shot, we need to add light by compensate + to make white white and not gray.
On the other hand, in the case of black wall or subject with dark background, our camera meter will think it is not bright enough and will over expose the shot, we need to subtract light by compensate – to make black black and not gray.