Source: SC Psychological Enterprises Ltd

Visual acuity is a measure of how small a detail that is visible can be, and still be identified by a person. The most common way of testing vision is to use an eye chart, (the kind that you see in an office of an optometrist with the big E on the top row). This is known as a Snellen Eye Chart, because it was designed by Hermann Snellen in the late 1800's. It uses symbols that are formally known as "optotypes". These have the appearance of block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font collection. They have a particular, simple geometry in which in which the sizes of lines, gaps and white spaces are carefully determined so that if you can't see them clearly enough, the letters become confusing and difficult to read. The letters (and the white spaces and gaps that define them) get progressively smaller as you move down the chart. The line with the smallest letters that you can read is a measure of your visual acuity or more specifically, your eye`s detail resolving ability.

The actual number assigned to measure your acuity is based on how well you compare to a person known to have normal 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 measuring 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 normal vision can read them at 40 feet, then your vision is 20/40 (or 6/12).

Obviously we can't get a dog to read rows of letters for us, so we use another technique for determining his acuity. In this test we want the dog demonstrate to us that he can see the details that define a pattern. The pattern we use for dogs is a simple grid made up of equal sized black and white vertical stripes. We put this pattern up next to a uniform gray pattern. If the dog`s vision is good enough then he can pick out the striped pattern and if he does so he gets a treat. If he picks the gray he gets nothing. Next we test him with stripes that become narrower and narrower. This is equivalent of making the letters on the eye chart smaller and smaller. Eventually the stripes will be so thin that the dog's visual acuity will not allow him to see 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 the card with the uniform field of gray. 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.



The best set of measures that we have to date on this kind of acuity measure is from a dedicated Poodle, tested in Hamburg, Germany. Still his acuity was quite poor and he was only able to make out patterns that have stripes that are nearly six times wider than the minimum that humans can make out. Converting this result to the more usual measures the dog seems to have a visual acuity of only 20/75. This means that a pattern that a dog can barely recognize at 20 feet (6 meters) is actually large enough for a person with normal vision to identify at a distance of 75 feet (23 meters). To give you a feeling about how poor this vision is, you should know that if your visual acuity is worse than 20/40 you would fail the standard vision test given when you apply for a driver's license in the United States and would be required to wear glasses. A dog's vision is considerably worse than this.

Don't let these numbers fool you, however. Although the dog's visual acuity is considerably less than that of a normal human, a lot of information is still getting from his eyes to his brain, even though the focus is "soft" and he won't be able to make out many details. The 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.

Stanley Coren is the author of many books including: The Modern Dog, Why Do Dogs Have Wet Noses? The Pawprints of History, How Dogs Think, How To Speak Dog, Why We Love the Dogs We Do, What Do Dogs Know? The Intelligence of Dogs, Why Does My Dog Act That Way? Understanding Dogs for Dummies, Sleep Thieves, The Left-hander Syndrome

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