Previously there was a general opinion that horse’s vision was kind of blurry and that was one of the reasons why horses seemed so distracted at times. Today we know better, even if much is yet to be studied and explained.
Horses’ ability to see small detail, or visual acuity, has been the subject of several scientific studies with quite different results. Since we cannot make horses reel off the letters from a Snellen chart it is not really the same requirements as when measuring human visual acuity.
According to the latest studies it seems as horses’ visual acuity is slightly inferior to ours, but clearly better than other mammals like dogs and cats. Normal human acuity is called 20/20 vision, while horses would have a 20/30 vision. Take a look at this Snellen chart:
If we do some calculation we can say that, under certain conditions, a human with 20/20 vision can distinct between details 29mm wide at a distance of 100 meters, while horses can distinct between 44mm wide details. It is not a very big difference. Remember that human vision has been studied on billions of people while only a very small number of horses have been tested using very different methods not easily comparable, so the results may have been significantly underestimated concerning horses visual acuity.
The horse has a horizontal linear visual band (compared to the human point like macula) within the retina with a high concentration of photoreceptor cells, which gives the best acuity. Maximum acuity (sharpness) for us humans occurs in the center of our view, but only seven degrees from the center the acuity is down to 15%. At the edge of the binocular field the acuity is down to 4%. A sign that is easy to read when you look right at it is impossible to read if you focus beside it. We feel as if we have good acuity all over our visual field, but that is just an illusion. This feeling is probably the same for the horse; wherever they focus their attention the acuity is good. How horses’ acuity drops when off center is even more difficult to study, but it would be interesting to know how the horizontal linear visual streak of the horse performs as compared to our point like macula. Maybe horses have a larger field of fairly good acuity than we have?
Both humans and horses have what is called accommodation; we can change the shape of the lens to focus an image on the retina. Accommodation is primarily important when focusing on things up close because then the muscles that shape the lens contracts and to focus at a distance the muscles relax. An old misunderstanding is that horses focus by tilting their head to focus an image along a so called ramp retina, but this is not correct. When it first was discovered that horses did have accomodation it was found to be much weaker than ours, but now it has been recognized that their lens require less change to keep the image focused than the human lens does. The behavior of moving the head which was thought to be a way to try and focus is now only associated with binocular vision, as discussed in blog post #3.
It is of course likely that there are variations in acuity between individual horses, just as among humans. Refractive errors among horses have been studied on a limited number of horses so there are few good statistical facts about that, but it seems to be more common with hyperopia (bad short range acuity) compared to myopia (bad long range acuity) which is quite logical among most mammal species.
In one study of fifteen domesticated horses (Farrall and Handscombe) one in five horses was myopic and two out of five hyperopic. Myopia is undeniably a disadvantage for any animal in the wild, prey or predator; any heredity towards myopia must be ruled out by natural selection. To focus on objects closer than one meter, the muscles which work to shape the lens get very strained. Doing so often and over long periods of time may lead to difficulties for the muscles to relax, which is needed when focusing on long range. There are suspicions that the domestication of the horse could have made myopia more common, but I have not found any scientific study which supports that. What I do know is that our Nokota horses seems to have excellent acuity on long range, as they can spot a fox at the same (if not farther) distance as I myself only just barely can with a 20/20 vision, with the reservation that it also involves other visual properties as discussed in previous sections of this blog series.
As all photographers know; the smaller the focal ratio number, the bigger the focal diameter and the more light enters the camera, and the smaller the depth of focus. The focal ratio is the focal length divided by the aperture diameter, in this case the pupil diameter.
The human focal length is typically 16,7mm and we can adjust the pupil diameter between 2 and 7 mm (in order to let in just the right amount of light). This gives the human eye a focal ratio span of 2,4 to 8,3.
The horse has a focal lenght of typically 25mm. Since their pupil is more like a horizontal oval or ellipse we use the average, or equivalent, diameter which is 30mm when fully open (2008 Roth et al). This gives the horse eye focal ratio 0,83 when the pupil is fully diluted (as compared to the human 2,4). I have not found any figures on the size of the minimum pupil so we have to leave that out unfortunately. However, the third eyelid which can shade the retina from light coming from above as well as the shape of the pupil which gives a horisontal field of view, means that the horse eye do not need the same high end focal ratio as we do.
This exercise tells us that horses probably can’t reach the same depth of focus as we humans can. It does not mean they have less good acuity, only less depth of focus. Meaning that when they focus on an object, the background, as well as closer objects, seems more blurred than we would experience, if all other properties are given equal. This may not be a disadvantage for the horse since it would make the focused object stand out against the surroundings. Note that horses’ eyes need much less light than ours to make a perfect picture, so if our pupil need to open fully the horse’s pupil may still be fairly closed, so in that particular case both horse and human would roughly enjoy the same focal depth.
The next and final part of this series on Equine Visual Perception is where we draw the conclusions about equine visual conception and discuss how to apply them on our everyday life among horses, so stay tuned.