 This is due to the contraction of the pupil, instead of the usual dilation (mydriasis) that is undergone when a normal, untrained eye, accustomed to viewing in air, is submerged.Visibility is a measure of the distance at which an object or light can be discerned.The theoretical black body visibility of pure water based on the values for the optical properties of water for light of 550 nm has been estimated at 74 m.

The standard measurement for underwater visibility is the distance at which a Secchi disc can be seen.