Do Dogs Need Glasses?

Do Dogs Need Glasses?

Just how good is a dog’s visual acuity? First of all, let’s see how we measure visual  acuity. If we use a typical eye chart (the kind that you see in the office of an  optometrist, with a big E on the top row), the smallest letters that you can read  accurately determine your visual acuity. If you are tested at a distance of 20 feet  and can read the same line of letters that a person with normal vision can read at  20 feet, then the Snellen measure of your vision is 20/20 (or 6/6 if you are mea-  suring the distance in meters). If your vision is not that good, then you will need  the letters printed much larger to read them at that distance. Thus, if the letters that  you can just barely read correctly at 20 feet are large enough that a person with nor-  mal vision can read them at 40 feet, then your vision is 20/40 (or 6/12).

Since we can’t get a dog to read for us, we use another technique to determine  his visual acuity. In this test we teach a dog to select a pattern made up of equal-  sized black and white vertical stripes instead of a patch that is a uniform gray with  no stripes. If he picks the one with the stripes he gets a treat, but if he picks the  gray he gets nothing. He is then tested with stripes that become narrower and nar-  rower. Eventually the stripes will be so thin that the dog’s visual acuity will not be  able to determine that they are there. At this point the stripes blur and smear in the  eye, and the card with the stripes will look the same as a card that is a uniform field  of gray with no stripes. When the size of the stripes just arrives at this point, we  have reached the limits of the dog’s visual acuity. The size of the stripes that the  dog can see can be converted to the same Snellen acuity measure that we get from  an eye chart used to test people.

In actual practice, dogs appear to get a bit frustrated in the testing situation  when the lines get very thin and near the limits of acuity. Rather than carefully  studying the patterns the way that a person might in order to make out the appar-  ently blurry lines, the dog simply gives up and chooses randomly. It seems that  dogs just don’t rely on their vision as much as humans, and, after all, even by just  guessing, the dog will get a treat at least half of the time. The best performance to  date on this test is from a dedicated poodle, tested in Hamburg, Germany. Still his  acuity was quite poor and he was only able to discern patterns with stripes that  were nearly six times wider than the minimum that humans can perceive. Con-  verting this result to the more usual measures, the dog seems to have a visual acu-  ity of only 20/75. This means that an object that a dog can barely see at 20 feet (6  meters) is actually large enough for a person with normal vision to see at a dis-  tance of 75 feet (23 meters). If your visual acuity were worse than 20/40, you would  fail the standard vision test given when you applied for a driver’s license in the  United States and would be required to wear corrective lenses. A dog’s vision is  considerably worse than this.

Don’t let these numbers fool you, however. even though the focus is “soft” and he won’t be able to  make out many details. The overall effect is something like viewing the world  through a fine mesh gauze or a piece of cellophane that has been smeared with a  light coat of petroleum jelly. The overall outlines of objects are visible, but a lot of  the internal details will be blurred and might even be lost.

Knowing something about the visual acuity of dogs helps to explain some ca-  nine behaviors that otherwise seem incomprehensible. For example, there were  occasional times when I would slowly walk out of the house when my dogs were in  the yard, only to have them stop and stare at me. As I continued to move toward  them at a slow speed, they would take a few nervous steps and might even hunch  down in a slightly defensive posture. It seemed quite clear at such moments that  they were not quite sure who I was. When I spoke to them, they immediately  seemed to relax and galloped toward me, with my flat-coated retriever, Odin, doing  his usual goofy greeting dance. Let’s change the scenario slightly. I again walk out  of the house to find the dogs in the yard, but now I am wearing the broad-brimmed  western style hat that I usually wear outdoors. At the moment Odin catches sight of  me, his usual response is to dance his way over to say hello, quickly followed by  the other dogs in the yard.

Why do they fail to recognize me in the first instance yet immediately identify  me in the second? In the first case the dogs had to discriminate who I was based  upon smaller, less distinct features, such as the shape of my eyes, nose, and  mouth, which are blurry to them at best. However, in the second instance there is a  distinctive outline to my shape, provided by my hat, which is a big enough visual  feature to be easily seen. No one else in my family has chosen the same style of  headwear, so this larger visual feature is quite easily recognized even given the soft  focus of their eyes.