Reflected and incident light and in camera sensors.
The difference between the two is very simple, yet the results will vary wildly depending on what settings you use in camera and whether you opt for a light meter.
Your camera will measure reflected light and its’ censor can be easily fooled. The following three shots are of black, white and mid grey surfaces using full auto mode and are without subject focus. I have also added some Photoshop sketches to show how the histograms looked in camera.
As you can see from the above images and histograms there is very little difference. The difference in the histograms has occurred due to the tonal difference as a result of the subjects’ surface coating and the materials they were made of. The conclusion is that your camera’s censor cannot be relied upon at all times and to achieve your desired exposure your will need to adopt different techniques. May be using a handheld metering or gaining multiple readings of the scene, to achieve what you want. There is no such thing as one correct exposure and this will also depend upon your personal photographic voice and what you are wanting the photograph to say.
Manual adjustments were made to the following images to place the tone at the correct point on the histogram. This needed the sensor to be fooled, by either under exposing (black) or over exposing (white) the shots to gain correct placement.