If you’ve never learned the technical details of what “middle gray” and “18% gray” are, check out this 6-minute video by Filmmaker IQ. John Hess shares the math and science behind the concepts to demystify them.
Middle gray is simply the halfway point between white and black to us perceptually. Our perception of color and lightness isn’t linear, and due to its history of use in printing with black ink on white paper, “middle gray” has been typically defined in photography and photo printing as a gray that reflects 18% of the light that hits it.
“Most scenes reflect just 12% to 13% of incident light falling upon them. Therefore, the camera light meter assumes a 18% gray level,” Wikipedia states. “This can easily be observed when one relies solely on the exposure given by a camera with a built in light meter when taking a snow scene – the image will come out dark. Using a 18% gray card as an expose guide will mitigate this error.”
If you enjoyed this explanation by Hess, be sure to watch his 30-minute lecture on the science of exposure.