18% gray is considered the middle exposure level of a scene and is the industry standard used when adjusting the exposure of a shot.

Since human perception is non-linear and logarithmic–giving more bias toward moderately bright image information–humans perceive a diffuse surface where 18% of incident light is reflected to be mid-level brightness, as opposed to surface with 50% reflectivity.

18% gray cards are manufactured to precise standards and are an essential part of a cinematographer’s kit.