Transducin activates an enzyme that breaks down cyclic GMP . Interneurons are relieved of their normal inhibition. This, in turn, relieves the inhibition of the spontaneous firing of the ganglion cells to which they are connected. So the retina is not simply a sheet of photocells, but a tiny brain center that carries out complex information processing before sending signals back along the optic nerve. In fact, the retina really is part of the brain and grows out from it during embryonic development.
Rod vision is acute but coarse.
Rods do not provide a sharp image for several reasons. So if only a single rod is stimulated, the brain has no way of determining exactly where on the retina it was. However, rods are extremely sensitive to light. So although rods provide us with a relatively grainy, colorless image, they permit us to detect light that is over a billion times dimmer than what we see on a bright sunny day. Although cones operate only in relatively bright light, they provide us with our sharpest images and enable us to see colors.