A dog's sense of sight is different to ours. They have a wider field of vision because their eyes are set further toward the sides of their head.
However, the field of vision from each eye does not overlap as much as ours do, so they do not see as distinctly or in focus especially close up.
The dog's sense of sight enables them to be very good at noticing movement and often see things at a distance before humans do. If you are training your dog, most voice commands will be given along with hand movements for this reason, and dogs will learn to respond to these movements without the need for any voice commands.
Dogs also have a third eyelid. This is a layer that protects the eye from irritants like dust and is sometimes noticeable in front of the eye.
The dogs retina refreshes more quickly than the human retina does, contributing to the dog's superb ability to perceive moving objects and the power to see shapes and details at a far greater distance than humans.
Dog's eyes function well in dim light, they have good night vision, but they can't see in complete darkness. The dog's eyes are sensitive to light and movement. They are able to dilate the pupils to take in any light around them in order to see better at night. The tapetum lucidum, located behind the retina, is a luminescent layer which causes the scary greenish look to the eyes that shows up in low light and sometimes in photographs.
We can distinguish many more colours than dogs, but science has recently proved that dogs can actually see different colours and not only certain shades of black and white as was first thought.
A dog's sense of sight can become impaired by cataracts. The lens of the eye can become cloudy and block light from getting to the retina. It can start in a small area but eventually cover the complete eye resulting in blindness. Cataracts can be caused by diabetes, injury or inheritance , but the only known cure is surgery.