Both dogs and cats have adaptations for improved vision in poor lighting conditions. One reference indicates that the minimum threshold of light for vision in cats is approximately 6 times lower than that for normal human beings. Dogs are considered to be somewhere between cats and humans in regards to night vision. What are these adaptations? It is the rod photoreceptors of the retina that are necessary for vision in dim light. In dogs, the central 25% of the retina consists predominantly of rods. For comparison, this region consists predominantly of cones (for color vision and vision in bright light) in humans. Also, most dogs and cats have a reflective structure called a tapetum in the top half of the back of the eye immediately below the retina. This structure is what gives these animals their "eye shine" (reflectivity) in the dark. After light passes through the retina, it is reflected from the tapetum back onto the retina so that there is a second opportunity for the retinal photoreceptors to be stimulated. There is a price for this feature, however. Light scattering can reduce the ability of the eye to resolve the fine details of the image. When it comes to vision, there are trade-offs. Dogs are better at detecting moving objects than stationary ones.
Because most dogs' eyes are more laterally placed than are the eyes of humans, they have a resultant wider field of view. This provides for a greater ability to scan the environment. Again, there is a trade-off because the more frontal placement of humans' eyes allows for better binocular vision than found in dogs. In dogs, both the field of view and the degree of binocular vision vary by breed because of breed differences in position of the eyes. Additionally, depth perception in dogs can be impaired by the nose, which also varies by breed.
Question: Are dogs near-sighted or far-sighted?
Answer: In most cases is neither! In a classic paper, it was shown that the vast majority of dogs have excellent vision with neither significant near or far-sightedness. There are exceptions. In the study mentioned, 2 breeds showed a higher incidence of myopia (near-sightedness): German Shepherd Dogs and Rottweilers.
Question: Do dogs (and cats) really see only in black and white?
Answer: No! That is a common misconception. Cone photoreceptors are the cells in the retina responsible for color vision. They are present in dog and cat retinas. Although dogs and cats do have some color vision, there are species differences in color vision. Humans with normal vision have 3 types of cones. Dogs and cats have only 2 types of cones so they do not experience the same spectrum of color vision that we do. Color vision in dogs and cats is called dichromatic versus trichromatic color vision in people. It is thought that dogs can see blues and yellows the best and that they can distinguish reds from blues but that they have difficulty distinguishing reds from green. They are considered similar to red-green color-blind humans.
Question: What happens to my pet's vision if he or she loses vision from one eye?
Answer: One consideration is depth perception. It is best where the visual fields of the two eyes overlap, when both eyes have normal vision. Keep in mind that it is not nearly as good in dogs or cats as it is in humans. In dogs, it is estimated to be 30-60 degrees compared to 140 degrees in humans. In dogs, the nose affects depth perception and therefore depth perception should be best when the dog looks straight. Monocular (i.e., one eye) depth perception is possible. Head movements can give cues as to the depth of different objects and there are other cues that indicate depth such as relative brightness, etc. There is no doubt that vision is best when both eyes are visual. However, dogs and cats with vision from only one eye can still manage and many such affected cats still jump successfully!
Question: How will my pet's vision be if the lens needs to be removed (i.e., due to dislocation or cataract surgery)?
Answer: Without the presence of a lens to focus (accommodate), the refractive state of the eye changes to far-sightedness (hyperopia). Caution should be used, however, when trying to compare the effects on animals to humans. The ability of the canine lens to accommodate is not anywhere near that of humans. But another reason that aphakic dogs cannot be compared to aphakic humans, where aphakia is devastating to vision, is that dogs do not have the area of retina called a fovea so the impact on vision is not the same. That being said, the ideal situation after lens removal is artificial lens replacement as an aphakic dog is about 14 D hyperopic. However, this is not always possible so your pet might end up what is called aphakic, which means no lens present. Many pets still function quite well despite aphakia though obviously this situation is not optimal for vision.
Question: How is a dog's visual acuity?
Answer: Compared to humans, it is better in dim light but worse in bright light. One paper cites the likely visual acuity in dogs as 20/75. Remember that the visual challenges for a dog are different than they are for people as they are not required to do the same tasks as are people