Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans (deuteranopia).[109][110][111][112] Dogs are less sensitive to differences in grey shades than humans and also can detect brightness at about half the accuracy of humans.[113] The dog's visual system has evolved to aid proficient hunting.[109] While a dog's visual acuity is poor (that of a poodle's has been estimated to translate to a Snellen rating of 20/75[109]), their visual discrimination for moving objects is very high; dogs have been shown to be able to discriminate between humans (e.g., identifying their owner) at a range of between 800 and 900 m, however this range decreases to 500600 m if the object is stationary.[109] Dogs have a temporal resolution of between 60 and 70 Hz, which explains why many dogs struggle to watch television, as most such modern screens are optimized for humans at 5060 Hz.[113] Dogs can detect a change in movement that exists in a single diopter of space within their eye. Humans, by comparison, require a change of between 10 and 20 diopters to detect movement.[114][115] The physiology of a dog ear. As crepuscular hunters, dogs often rely on their vision in low light situations: They have very large pupils, a high density of rods in the fovea, an increased flicker rate, and a tapetum lucidum.[109] The tapetum is a reflective surface behind the retina that reflects light to give the photoreceptors a second chance to catch the photons. There is also a relationship between body size and overall diameter of the eye. A range of 9.5 and 11.6 mm can be found between various breeds of dogs. This 20% variance can be substantial and is associated as an adaptation toward superior night vision.[116] The eyes of different breeds of dogs have different shapes, dimensions, and retina configurations.[117] Many long-nosed breeds have a "visual streak" a wide foveal region that runs across the width of the retina and gives them a very wide field of excellent vision. Some long-muzzled breeds, in particular, the sighthounds, have a field of vision up to 270 (compared to 180 for humans). Short-nosed breeds, on the other hand, have an "area centralis": a central patch with up to three times the density of nerve endings as the visual streak, giving them detailed sight much more like a human's. Some broad-headed breeds with short noses have a field of vision similar to that of humans.[110][111] Most breeds have good vision, but some show a genetic predisposition for myopia such as Rottweilers, with which one out of every two has been found to be myopic.[109] Dogs also have a greater divergence of the eye axis than humans, enabling them to rotate their pupils farther in any direction. The divergence of the eye axis of dogs ranges from 1225 depending on the breed.[114] Experimenta

ion has proven that dogs can distinguish between complex visual images such as that of a cube or a prism. Dogs also show attraction to static visual images such as the silhouette of a dog on a screen, their own reflections, or videos of dogs; however, their interest declines sharply once they are unable to make social contact with the image. Visual acuity (VA) is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain.[1] Visual acuity is a measure of the spatial resolution of the visual processing system. VA is tested by requiring the person whose vision is being tested to identify characters (like letters and numbers) on a chart from a set distance. Chart characters are represented as black symbols against a white background (for maximum contrast). The distance between the person's eyes and the testing chart is set at a sufficient distance to approximate infinity in the way the lens attempts to focus. Twenty feet, or six metres, is essentially infinity from an optical perspective. (The difference in optical power required to focus at 20 feet (6.1 m) versus infinity is only 0.164 diopters.) In an eye exam, lenses of varying powers are used to precisely correct for refractive errors. A pinhole is also used to largely correct for refractive errors. English alphabet letters are typically used (as in the classic Snellen chart) as most people will recognise them but other symbols (such as a letter E facing in different directions) are also used. In the expression, 20/40 vision, the 20 is the distance in feet between the subject and the chart. The 40 means that the subject can read the chart as well as a normal person can read a chart that is 40 feet away. This is calculated by finding the smallest optotype they can identify and calculating the distance at which it has a visual angle of 5 arcminutes. The letters in the lowest line in the Snellen chart are composed of lines that are separated by a visual angle of one arc minute. This corresponds to lines that are spaced only 1.75 mm apart. Each succeeding higher line on the Snellen chart contains letters whose lines are separated by correspondingly larger visual angles. A person who can correctly identify letters on the lowest line is able to discern individual lines that are separated by a visual angle of one arc minute. The metric equivalent of 20/20 vision is 6/6 vision. At 20 feet or 6 metres, a human eye with nominal performance is able to separate lines that are one arc minute apart (equivalent to lines that are spaced 1.75 mm apart). A vision of 20/20 is considered nominal performance for human distance vision. A vision of 20/40 is considered half as good as nominal performance. A vision of 20/10 is considered twice as good as nominal performance.