Equine Ophthalmology

The horse has a total visual field of nearly 360°, meaning a horse can just about see its tail with its head pointed forward. A small frontal binocular field of 65° is present. The horse retina is adapted for detection of movement because it contains significant numbers of large-diameter ganglion cells. The nasal extension of the retina, the laterality of the eyes, and the horizontal pupil facilitate tremendous peripheral vision for the horse standing with its head up. The horse has weak accommodative ability of the lens and therefore has limited near focus capability. The horse uses both eyes until an object approaches within 3–4 ft, when it is forced to turn or lower its head to continue to observe with one eye. Horses need to accommodate 2 D to maintain a focused image on the retina. Mean refractive error of horses is 1.0 D. This seems to be similar to 20/33 vision on the Snellen chart. Horses have poor acuity in the peripheral retina. Rods significantly outnumber cones, indicating the high capacity for night vision in horses, but cones are present in the horse retina with the capacity for color vision. Horses see blues and yellows but may have more trouble with longer wavelength colors such as red. 1