The lens is very flexible in most birds and can change shape readily - birds need to focus near and far and change rapidly. The retina is the sensitive layer of the eye that absorbs the incoming light, senses it, integrates the information in it, and sends this information to the brain. The entire retina is thicker than that of mammals and the rods and cones are more abundant. Cones are for color and rods are for black and white and dim light. The distribution and density and proportion of rods to cones varies with the species of bird.
Diurnal birds have retinas dominated by cones while nocturnal birds have mostly rods.
Many hawks and owls have more sensory cells in the upper half of the eye that receives more light when the birds look down. Any part of the retina that has a denser concentration of receptor cells perceives sharper images. One such area of densely packed cells is called a fovea . Not only are the cells denser in that area but the pit shape serves to refract the light so that a larger image is formed. Overlapping of the visual fields of the two eyes produces binocular vision and depth perception.