In addition, there is a reflective layer under the retina called the tapetum which serves to intensify vision in dim light. While dim light vision is enhanced by the tapetum, scattering of the reflected light may result in reduced acuity. Although it is commonly believed that dogs and cats see only in black and white, recent evidence suggests that animals may have some degree of useful color vision. The perception of color is determined by the presence of cone photoreceptors within the retina. Behavioral tests in dogs suggest that they can distinguish red and blue colors but often confuse green and red.
Acuity is the ability to focus so that two objects appear as distinct entities.
This is the value that is measured in people using an eye chart. This means that, at 20 feet, a dog can distinguish an object that a human could see at 90 feet. One of the most common diseases which can affect the clarity of the eye is formation of a cataract within the lens. Cataracts in dogs are most often inherited and may affect dogs at any age. As the lens becomes progressively opaque, an animal's vision deteriorates so that only light and dark perception exists.