One of the most commonly circulated dog "facts" is that all dogs are colour blind. Like most pseudo-scientific hear-says, this well-known "fact" falls short of being entirely true. Interestingly though, it does hold a smidgen of truth, as dogs aren't quite capable of seeing the world exactly as we do.
A dog's vision isn't just restricted to perceiving shades of grey, black and white. The retina, much like a human's, contains two main kinds of photoreceptors: rods and cones. Cones are responsible for colour perception, and dogs have only two types of cones in their retinas, as opposed to humans who have three.
This means a dog's vision can be safely equated to that of someone with red-green colourblindness, according to Jay Neitz, a colour vision scientist at the University of Washington. Like people with red-green colourblindness, dogs perceive colours differently than humans with normal colour vision.
Human Vision Dog Vision
Red Dark brown
Neitz's research indicates that dogs, like colourblind people, may use certain cues to distinguish one colour from another. There are usually good cues to help them figure it out: for example, red objects tend to be darker than green objects. So, if it's a dark apple, a red-green colour-blind person would know that it's probably a red one, and if it's a lighter apple, it may be a Granny Smith.
A Dog's Super-Sight
However, it is essential to understand that although dogs don't perceive colour as well as we do, it doesn't mean we have superlative eyesight in comparison. Rods, the second type of photoreceptor cells present in both human and dog retinas, are responsible for light sensitivity and motion detection.
Dogs' retinas actually contain a higher concentration of these, which allows them to have a higher acuity of vision in darker environments. It makes them much better than us at picking up subtle movements in very dim light. This is further enhanced by the layer found behind their retinas called a tapetum, which reflects light that isn't captured by the rods and cones back onto the retina – humans don't have these. The tapetum allows the smallest amount of light/movement to be captured by the eye, helping them to hunt better at night.
The green glow captured by the camera is the tapetum found behind the retina of the dog's eyes