Visual acuity is the ability to see the details of an object separately and unblurred. Acuity is measured in “cycles per degree”, which means how many lines you can distinguish as being separate in a degree of the visual field. Humans see 30 cycles per degree, horses 18, dogs 12 and cats 6. Acuity in dogs is 0.4 times that of people, 0.67 times that of horses, and twice that of cats. Acuity in cats is 0.2 times that of people, 0.33 times that of horses, and 0.5 times that of dogs. If normal human vision is 20/20, then that of the dog between 20/50 to 20/100, the horse 20/33, and that of the cat is 20/100. However, it is difficult to measure acuity in animals so studies have often shown wide variations in results.
The picture demonstrates the idea of acuity and how it differs between humans (top) and dogs (bottom). Humans can distinguish a lot more “cycles” in the same picture. To a dog, the upper image would be a gray blur with no alternating lines.