"How well can my pet see?" is a common question pet owners pose to veterinarians. They almost universally assume that human
vision is best. In the ways that laypeople mean the question, this is probably a fair statement. Though they may use other
terms, clients are usually referring to visual acuity and "color vision," and humans are superior in both these regards.
There are a number of other factors that contribute to functionality of vision, however, which will be discussed, including
pupil shape, globe position, photoreceptor composition, and presence/ absence of tapetum. It is the lecturer's opinion that
adaptations exist within species that allow them to function at their best- within their own environment and lifestyle. The
dog, cat, and horse are used here to demonstrate these adaptations.
Visual acuity is used to describe the maximum resolving power of the eye, or ability to see detail. Its what optometrists
measure when calculating your prescription. Emmetropia is the term used for normal visual acuity- hopefully the way you see
when wearing your glasses or contact lenses. Ammetropic individuals are either near-sighted (myopic) or far-sighted (hyperopic).
The common tests used to evaluate visual acuity in people record a value of 20:20 when emmetropia is present, which in simplest
terms means that they can discern from 20 feet the detail that they should at that distance. People with 20:30 vision can
only discern from 20 feet the sort of detail that a person with normal vision can from 30 feet. Since our patients cannot
read- or communicate that they correctly read- an eye chart, this test becomes useless. Nonetheless, other means, including
retinoscopy and mathematical formulas, have been used to estimate visual acuity in domestic animals. Horses are estimated
to have acuity of about 20:33, dogs of 20:75, and cats of 20:100. In other words, these three species have vision that is
hyperopic (near vision is relatively poor) when compared to human vision. However, they appear to have less individual variation
in visual acuity (so most dogs are roughly emmetropic for dogs). This is advantageous to veterinary ophthalmologists, who
can expect most of their patients of the same species to benefit substantially from placement of a standard artificial intraocular
lens (IOL) after cataract extraction. The power of IOL required to return an average dog to emmetropia (41 Diopters) is different
from that required for the cat (52 Diopters), however, because the contribution of the lens to vision varies among species.
Although the lens is very important for "fine focus vision," and lens replacement when possible has become standard in the
veterinary ophthalmology community, it is important to note that eyes without lenses are not blind. In fact, the cornea is
a much more important contributor to vision than the lens. Aphakic eyes are substantially hyperopic/far sighted (by 14 Diopters
in the dog), so their near vision is blurry, but even bilaterally aphakic dogs without other substantial ocular abnormalities
function very well. For an image to focus on the retina, roughly parallel light rays entering the eye must undergo a series
of refractions to reach the appropriate retinal layers in very precise arrangement such that a sensible image can be sent
to the brain for processing. In an individual with perfect vision, the light rays entering the eye converge at exactly the
right level. With hyperopia (such as seen with aphakia), near images are formed behind the retina, while with myopia, far
images form in front of the retina. In both cases, the affected images lack clarity and are seen with "fuzzy borders." The
cornea is the most powerful refractive structure of the eye, while the lens is the second most important, and the vitreous
is much less important. Opacities at any layer within the normally clear visual axis (cornea, aqueous, lens, vitreous) cause
scatter of light rays, and degrade the quality of the image formed.