When you leave a brightly lit room for a garden at night, the scene may initially look almost black. After a few minutes, trees, paths and other outlines gradually emerge. This process is called dark adaptation. The pupil widens in dim conditions and admits more light, but that explains only the earliest part of the change. Visual sensitivity continues to improve afterwards.
The retina has two main kinds of photoreceptor. Cones operate in bright conditions, respond relatively quickly and support detailed colour vision. Rods are more sensitive to faint light but do not provide normal colour discrimination. Bright light leaves photopigments temporarily unable to absorb another photon. In darkness, those pigments progressively recover. Cones provide an early, limited improvement; rods then become the main pathway for low-light vision. Adaptation is therefore a process, not a switch.
This also helps explain why dim objects often appear grey and may be easier to see when you look slightly beside them. The central fovea is densely populated by cones, while rods are more numerous outside the centre of the retina. The central point is that the eye does not merely turn up the brightness of an image. It changes the relative contribution and sensitivity of different photoreceptor pathways. The rate of adaptation varies with previous illumination, age and eye condition, so this everyday effect should not be used for self-diagnosis.
https://www.aao.org/eye-health/anatomy/night-vision
https://openstax.org/books/introduction-behavioral-neuroscience/pages/6-2-the-retina
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