Dog Vision

Friday, May 25, 2012

I acquired a new lens a couple of weeks ago, and yesterday I put it through a battery of tests. Along with pointing it at a brick wall and banging off a few photos to see how well it resolves tiny nooks and crannies, I decided to take photos of Jazz, our dog, lying beside the window in our dining room—a real-life situation. The lens is sharp, produces pleasing color, and it renders straight lines true. (The table leg visible in the lower right-hand side of the frame is actually crooked.)

The lens is a 50mm macro prime. It utilizes an optical design that has been around for over thirty years. Its main purpose is for copying documents and photographs on a camera stand. Being a macro lens, it is able to focus on items as small as a postage stamp so that it fills the entire frame. This lens is also perfectly well suited for general photography. It is not fancy. It is not sexy. Unlike a zoom lens, it simply stays put at one focal length—50mm. In the days of 35mm still photography (think Leicas and old Nikons loaded up with Kodachrome), photographers referred to a 50mm lens as a "normal lens" because the field of view is the same as what a normal human eye sees. This is the first prime lens that I have used on my digital 35mm (dSLR) camera. It mimics the human eye in the same manner it would on a film camera.   

Dog Vision Facts

Normal vision for humans differs greatly from normal vision for dogs. Dogs see the world differently than humans for many reasons. Some are:

  1. Dogs stand closer to the ground than humans do. Whether a dog is a Great Dane or a Dachshund, they all view the world from a vantage point that is close to the ground.
  2. Since dog eyes are set further apart in the skull than human eyes, they have a wider field-of-view, a broader peripheral view, than humans.
  3. Dogs are only able to perceive two colors—yellow and blue. (Birds, humans, and apes are the only species known to have full color vision.)
  4. Dog eyes have only 20% of the color receptors (cones) humans have. The blues and yellows that they see are severely muted.
  5. Since dog eyes have so few cones, they have limited visual acuity. An average dog has 20/75 vision. This means that a dog is able to distinguish at twenty feet what the average person can at seventy-five feet.
  6. Dogs are farsighted. Dogs with long snouts are more farsighted than dogs with short snouts.
  7. Dogs have better night vision than humans do. Their eyes have more light receptors—rods. Furthermore, dogs' retinas, like cats, have a mirror-like coating (tapetum) that reflects light back onto the rods from behind. The tapetum is what causes dog and cat eyes to appear to glow at night when they glance at headlights and other point sources of light.

Dog Vision Simulation

The picture below approximates how a dog will see me from the vantage point near my dining room window. The image is fuzzy, the colors are muted, the field-of-view and aspect ratio are wide, and the view is close to the ground. The furniture up close is out of focus to simulate farsightedness.

Senior Dogs, Cataracts, and Vision

Jazz is 10.5 human years old. She is a senior dog. Her eyes are milky; she has cataracts. The picture below is a close approximation of how I think she sees.

To Be Continued

I plan to blog more about how dogs see. Stay tuned. My next installment on dog vision will discuss motion detection, depth perception, flicker rate, and more on snout length and nearsightedness.



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