It may be restricted in some individuals as the length of head and size of muzzle and nostrils increase. The ability of the horse's eyes to adjust to changing light conditions is much slower than in humans. When a horse moves from an area of bright light to one of dim light, or vice versa, the eyes of the handler adjust more rapidly to differences of light intensity than those of the horse. Therefore, one should give the horse some time to adjust to differences in light when going into unfamiliar places such as stalls, arenas, or trailers. Rods and cones are two photoreceptor cell types found in the eye.
Cones regulate day vision and function in color discrimination.
Rods are more light sensitive and function in low light intensity or in the dark. The human eye has a high percentage of cones whereas the eye of the horse has a high number of rods. Although it appears that the horse is not able to distinguish color it does have superior night vision. Whether or not horses can see color is not completely known. Some studies have demonstrated that horses may be able to differentiate shades of yellow and green but not red and blue.