It’s probably no surprise to most people who own a dog that they see better at night than their
owners. Several adaptations explain their better vision in low light. One is a larger pupil, the
opening in the iris that regulates the amount of light that enters the eyes. Although pupil size
can be as small as 1 millimeter in humans, dogs typically have a much larger pupil, allowing
more light to enter the eye. A second reason is that dogs have more light sensitive rods in their
central vision whereas humans have more cones which are used for daytime vision and color vision. But the biggest advantage dogs have is a structure in the eye called the tapetum, a mirror like membrane which reflects light back to the retina, giving it a second exposure. However, the tapetum does cause more scatter in normal daylight, degrading their vision from our normal 20/20, to about 20/80.
So despite a dog’s improved night vision, during the day they have a RUFF time seeing!