There are 3 photopigments for cones and one for rods. For all four, when light photons contact them, the photopigment breaks into its two constituents. This helps stimulate the receptors, which sends a message to the horizontals and bipolar cells. Then the receptor s chemicals re-combine the two constituents. The more light, the more the receptor s photopigment that is bleached.
Dark adaptation is the recovery of many photopigment molecules in the receptor.
Fixation is centering an image on the fovea. Fixation involves small tremors and drifting off the fovea, and quick corrections . Riggs et al used mirrors and a contact lens to project an immobile stabilized retinal image . The result was image details disappeared and the image fogged over . Therefore, we need retinal changes to maintain an image.