Focusing on the World

Focusing on the World

In a camera the image is brought into focus by moving the lens forward and back  so that the distance between the lens and the film is modified. In human beings  the image is focused by muscles that actually change the shape of the lens, making  it flatter when objects are far away and round and fat when objects are close. The  dog can’t change the shape of its lens as much as humans can, but there is some  suggestion that, like cats, dogs have a set of muscles that can make the eye slightly  longer or shorter to help focus the image. How efficiently the lens changes focus  has a direct effect on the visual acuity of dogs.

If the eye can focus the image exactly at the level of the retina, you will get the  best visual acuity that the eye’s optics allows (this most desirable state is  technically called “emmetropia”). If the image comes to a focus too soon (that is,  before it reaches the retinal surface) or too late (it really wants to focus on a point  beyond the retinal surface), this will result in a blurred image. Each of these two  types of failures to focus results in a different kind of visual problem. If the light  rays come to a focus too soon, this results in nearsightedness (called “myopia”),  which means that close objects can be seen clearly but far objects are blurred.

If  the light rays come to a focus too late, this results in farsightedness (called “hyper-  opia”), which means that distant objects are clearly seen but near objects are  blurred. It is possible to estimate the focusing ability of an eye using a device  called a “retinoscope,” which measures the location of the point where light comes  to a focus by using a beam of light that is passed through the cornea and crys-  talline lens. To do this in dogs, you need a cooperative dog and a skillful, patient  researcher.

Christopher Murphy and a team of researchers from the School of Veterinary  Medicine at the University of Wisconsin reported a study that used a retinoscope  to measure the focusing ability of the eyes in 240 dogs. These scientists studied  cocker and springer spaniels; golden, labrador and Chesapeake Bay retrievers; Ger-  man shepherd dogs, poodles, Rottweilers, miniature schnauzers, Chinese shar  peis, and several different terriers and mixed breeds. Most dogs had emmetropic  eyes, meaning that the focusing ability of the eye is appropriate for the size and  shape of their eyes. However, there are some exceptions. More than half of the Rot-  tweilers were nearsighted, as were the miniature schnauzers and German shepherd  dogs.

The fact that these effects occurred in specific breeds suggests that they are ge-  netic in nature. Selective breeding seems to play a role, as demonstrated by re-  searchers who tested a set of German shepherd dogs that came from a line of dogs  specifically bred to be guide dogs for the blind. Among these German shepherds,  instead of having a better than 1 out of 2 chance to be myopic, only 1 out of every 7

was nearsighted. This seems to suggest that we can produce dogs with specific  variations in visual ability in the course of breeding them for certain tasks. It is also  consistent with some reports that greyhounds, which have been bred to search the  distance for game to pursue, are actually a bit farsighted. Although this is a devi-  ation from perfect focusing of the eye, it is a deviation in the direction that makes  greyhounds’ eyes more suitable for their work, namely to sight distant game and  chase it down.