When you move from a brightly lit to an unlit/dark space, your eyes will need some time to get used to this new situation. At first, you will be able to perceive hardly any or no objects in the dark space. But after your eyes have had some time to get used to the new situation, you will be able to distinguish more objects. This adaptation process of your eyes is also called adaptation/habituation.
The process of adaptation in your retina is very different from neural adaptation. This is because the adaptation in your retina depends on the sensitivity of your photoreceptors. The adaptation process is entirely determined by the (maximum) sensitivity of the two photoreceptors located in the eye: the cones and rods. This process takes place in two stages: a so-called fast initial stage and a later, slower stage.
Both receptors begin to adjust their sensitivity as soon as the light goes out. However, the cones do this immediately with a much greater speed than the rods. Hence the so-called fast initial stage.
After a few minutes, the cones have reached their maximum sensitivity and they cannot adapt any further. Meanwhile, the rods have continued their adaptation process and reach the same level of sensitivity as the cones a few minutes after the cones have reached their maximum sensitivity. The rods, however, have not yet reached their maximum sensitivity and so the adaptation process continues for them. After another few minutes, the rods have also reached their maximum sensitivity. At this point, your eyes have adapted to the new situation.
Author: Caroline Benjamins (translated by Anneke Terneusen)