The retina and the Stiles–Crawford effects

Refraction by the anterior eye is essential for proper focusing of light in the formation of images of the exterior world onto the retina. However, it is the absorption by pigments located within the photoreceptor cells that triggers the visual system. Understanding light-photoreceptor interactions is therefore necessary to unravel the complexities in the last optical step in the eye prior to subsequent neural responses. In the human retina, there are two kinds of photoreceptors that are responsible for vision, namely, the rods and the cones. The rods are responsible for dim light (scotopic) vision, whereas the cones are responsible for vision in normal and bright light (photopic) conditions. The transition from pure rod to cone-mediated vision is a combination of the two (mesoscopic) whereby the visual system has an astonishingly large dynamical range that spans about 12 log units, most of which is accomplished by the retina as changes in pupil size account for little more than 1 log unit.