Our understanding of canine vision is imperfect as we only have a perception of what they see and not categorical evidence. The ability to see in very low light conditions enhances a dog's ability to function as predators. A dogs eyes have a highly reflective layer of cells on the retina called the tapetum lucidum. This is responsible for the bright shine of a dog's eye when in a bright light. A dog's eye can focus allowing normal vision at different distances. They appear to be only able to accurately focus on objects no closer than 50cm to 33cm of their eye. They will then compensate by using other senses such as smell or taste.
So what exactly can a dog see, and how well? In humans we can take an eyesight test and tell the optician what we can see and whether it is in focus. So how can we know about a dog's eyesight? The nature of vision is a subjective thing, it is not easy to state exactly how a dog sees but it is possible to examine eyesight in differing light levels, visual acuity, movement, depth perception and colour. Dogs have a greater visual ability in low light conditions and they are able to detect motion and smaller movements at a greater distance than humans can. Depth perception depends on the degree of binocular overlap, in dogs the presence of a long nose can reduce this overlap and they may have difficulty judging distances. Visual acuity refers to the ability to see the details of an object separately and unblurred. On the whole dogs have reduced acuity compared to man. Colour perception is thought to be similar to a human who is red-green colour blind. This means that they see blue and yellow well but have trouble with reds and greens.