How dogs see and how their vision works is radically different from human vision - 14
DOGS VISION COLOUR RANGE
People have trichomatic vision, meaning that they can see the whole colour spectrum. Dogs however have dichromatic vision meaning they can only see part of the visual spectrum. Basically the image below shows the differences between the colour perception of people and dogs and shows that dogs perceive everything from red to green as a similar yellow colour, of varying brightness.
The following image is from ref 2
There are technical reasons behind this to do with the number and type of rods and cones that people have in the back of the eye compared to dogs.
“The retina of the eye is lined with both rods and cones in humans and dogs. The rods are much more prevalent in both species, but even more so in the dog than the human. The rods are adapted to work best in low light and are used for motion detection. The central retina of the canine eye contains about 20% cones, while humans have an area of 100% cones called the fovea. The cones work best in mid to high levels of light and have the ability to detect color.” Ref 2
The graph below shows that the rod structure sits in the centre of the visual wavelength spectrum and perceives brightness, while the cones (3 types in people, 2 types in dogs) is responsible for the range of colours each can see.
Image reference for the spectrum and man photo below = ref 5
|Human Vision spectrum||How humans view a scene|
Dog Vision spectrum
How dogs view a scene
The image of the man above is correct in its colour range, however the dog 'resolution' information below suggests that the dogs image of the man would be in much less detail. A big part of this colour difference and light sensitivity differences shown above is that wolves are nocturnal and relied more on the ability to see in dark conditions than discriminate between colours to see food. Dogs are evolved from wolves and it is only since their domestication that tame dogs have become diurnal (or active during the day) to fit into human patterns.
DOG LIGHT SENSITIVITY
Humans have the advantage of colour differentiation, dogs can see in much lower light conditions. This is because while a humans eye has the central area of its eye devoted to CONES and fevoa (which aids visual resolution) the dogs central region of the retina is mainly ROD cells that see various shades of grey much better. The RODS activate in much lower light conditions.
This means dogs can see around four times better than humans in the dark and that dogs need only about one quarter of the light that humans do to see things at night when hunting.
Journal of the American Veterinary Association (JAVMA), 207, (12), 1623-1634, Dec 15, 1995
DOGS FIELD OF VIEW
Dogs' eyes are generally more ‘spread out’ than humans. Predators often have eyes set in the front of their head because they don’t have a big fear of something sneaking up behind them and eating them and it assists field of view when chasing down prey. However Dogs/ wolves are also part scavenger and strong pack animals, meaning that they also need to coordinate hunts between several of their pack, and be aware of the position of their pack members.
This has led to the dogs (wolfs) eye set-up to providing them with an exceptional field of view up to 270 degrees compared to humans which can see a maximum of 180 degrees. The human advantage being that with a larger vision overlap, we get a better depth perception than dogs.
DOGS VISUAL ACUITY, RESOLUTION
This is an area that humans hold an advantage. Resolution or Visual Acuity is the ability to resolve shapes over a distance. For instance it is not uncommon for a human to have 20/ 20 vision. This means at 20 feet they can see what a person with ‘good’ sight can see at 20 feet. However a dog typically has approximately 20/ 75 vision (range 20/50 to 20/100). This means that a human at 75 feet can see the same shape differentiation that a dog has to be 20 feet from an object.
You may like to remember this the next time your dog is a long way off (with its head much closer to the ground) and you call them and expect them to see you. At long distances dogs rely much more on their higher audio detection capabilities and movement detection to see humans.
The images below show the difference between human and dog acuity. Humans can see about 30 cycles (or lines) per degree (top image) while dogs can only view up to 12 cycles per degree. (bottom image) (ref 2). For instance if a dog and a human are looking at this page and you (the human) can just make out separate lines on the bottom image, then the top image would appear as being all blurred together to your dog and a uniform grey colour. The images below are from ref 2.
Acuity is affected by the size of the pupil, size of lens and cornea, and the arrangement of the rods and cones on the retina. The pupil, controlled by the iris muscle is able to expand and contract to let in different amounts of light. An animal such as dogs that is active in dim light, tends to have a large pupil. And lower depth of field. (Ref 2)
DOGS MOTION DETECTION and LONG DISTANCE VISION
Dogs have much better motion detection than humans, due to the number of rods in the centre of the back of their eye. This means that in low light conditions (such as hunting at night) dogs can both see in low light conditions as well as detect the smallest of movement from prey. While they will not be able to detect camouflaged animals as well as humans (dues to our higher colour range detection) they can easily detect the slightest animal movement.
The previous segment stated that dogs have lower visual accurity than humans and less depth of field (ability to judge distance). And it is the large pupils (designed to let in more light for night vision), that causes dogs to only see objects in the centre of the image to be in focus when looking over long distances. Everything else in the image appears fuzzy. Ironically, while dogs are not able to see in as fine detail as humans, dogs can see things over greater distances - especially if the object is moving. “A test of 14 police dogs found that dogs could recognise a moving object almost half a mile (900 metres) away… but only 585m if it was stationary (ref 7)
DOGS FLICKER FUSION FREQUENCY
A little known fact is that Dogs can see flickering light better than humans. (ref 1) This reference suggests that this may mean that dogs may see conventional television or cinema as a series of moving frames rather than a continuous picture. This is because ‘flicker fusion’ relies on our eyes being tricked into seeing still frames (as used by film, TV and digital video files) as a continuous picture when they are run together faster than our flicker fusion threshold. For humans the threshold is said to be around 16 Hz minimum (16 frames per second).
While the smoothness of the image seemed to be covered at 25 or 30 frames for TV and 24 frames for cinema, the brightness was perceived by some people to flicker. So an interlacing method was invented for TV essentially showing the same frame twice (first odd lines of a frame, then the even lines of the same frame) – hence rates of 50 or 60 Hz for TV and 48 Hz for cinemas. In Australia we use a 50 Hz power system so TV frame rates were designed to run at 25 frames per second (50 Hz interlaced) and the North American NTSC system, 60 Hz power means TV runs at 30 frames per second.
HOWEVER, the flicker fusion rate for dogs is estimated to be as high as 70 to 80 Hz. (ref 2, ref 8)
This means that on conventional TV, computers and cinemas, that dogs will perceive films to be like a very fast PowerPoint presentation rather than a fluid image. They will see the same image several times, before the next frame replaces it! Now it is noted that most digital TVs are now based on progressive scan systems (which superseded interface systems to remove eyes strain associated with interline twitter). In Australia digital TV’s commonly have a 100 Hz or 200 Hz refresh rate (four times multiple of the power frequency), but the underlying digital video still uses 24 frames per second.
What some TV brands do to proclaim a more ‘smooth’ picture is to interpolate between frames (ie make up multiple frame images between real frames) using complex algorithms. They essentially estimate what the frames should look like if they were originally recorded at 100Hz or 200 Hz. Ironically your dog is the only animal in the house with a high enough flicker fusion threshold that may be able to detect if these manufacturers got it right.
Of course it should be remembered that this heightened skill of dogs was developed to detect small movement changes in the dark. It is the proportion and the type of rods and cones that dogs have that are different to us, that enable this fast flicker detection.
DOGS THIRD EYELID
Dogs have a third eyelid also called the haw or nictating membrane. This eyelid automatically moves up and down, sweeping the eye clean on a regular basis, allowing for uninterrupted night time hunts through thick brush. While this eyelid is normally transparent (and so not easily seen) if the gland associated with it becomes infected the infection can be detected as a red swelling known as ‘cherry eye’.
In considering the difference between human and dog vision we need to remember that dogs evolved from wolves a maximum of only 15,000 years ago. Wolves are nocturnal meaning that they survive by hunting prey at night time. To do this they needed to see in low light conditions and detect motion very well. This is how the rods and cones in their eyes evolved which is similar to how our domestic dogs see today. Meanwhile humans are diurnal (day hunters) who evolved in high light conditions where seeing the full colour spectrum with a relatively high level of detail is important.
However the reality is that dogs have a highly coordinated sensory system, a super sensitive sense of smell that in conjunction with their highly evolved sight sense, still makes them a formidable (latent) predator.
Article by Bruce Dwyer. If you wish to use any of this information please refer to the article as a reference and provide a link to http://www.dogwalkersmelbourne.com.au
Ref 1 http://www.vetinfo.com/dogsee.html
Ref 2 http://www.uwsp.edu/psych/dog/LA/davis2.htm
Ref 3 http://www.psychologytoday.com/blog/canine-corner/200810/can-dogs-see-colors
Ref 4 http://watchingtheworldwakeup.blogspot.com/2008_11_01_archive.html
Ref 5 http://www.diycalculator.com/sp-cvision.shtml
Ref 6 http://houndbound.com/dogblog/2009/12/27/can-dogs-watch-tv/
Ref 7 http://knol.google.com/k/canine-senses-how-dogs-see#