From an anatomical and functional point of view, dogs reach optimal vision around the age of 4 months.  

Until that age, a puppy has blurred vision and, if you start training him earlier, offer the dog the chance to see the ball or the rag by repeating the stimulating movement. You should not, under any circumstance, correct a puppy or an adult dog for seeing a moving object too late or not at all. 

To understand how a dog sees, one should know that there are two types of photoreceptors in the retina:  the rods and the cones. Rods register low levels of light in black and white and are responsible for night vision. Through the dilation of the pupil, the eye allows for more light to reach the retina. Thus, a dog can see well in low light, but can’t see at all in complete darkness.

The other photoreceptors, called cones, are needed to see in colors and in good light conditions. Until recently, it was thought that a dog’s vision is limited to black and white and grey tones. Recent experiments have revealed, however, that the dog’s eye can perceive a few colors. The fact that dogs’ eyes shine in front of a car’s headlights is due to an opaque layer behind the photoreceptive cells called tapetum lucidum, which reflects the light.

Dogs have good binocular vision and a reduced visual field.  Image focalization is more efficient in dog breeds with frontal eyes, compared to those with lateral eyes. Being extremely sensitive to the slightest changes in the environment and having an exceptional peripheral vision, dogs are efficient in visually scanning territories and even humans’ emotional states. A dog’s vision is good enough to match the standard European TV broadcasting system, which consists in 625 lines per frame. In other words, the dogs in Europe can watch TV programs without any difficulty, while the dogs in the USA, where the standard consists in 525 lines per frame, can only see moving dots and lines.