Celebrating World Animal Month: Multifocal Visual Systems in Animal Eyes

GOLDEN, CO–Every year, October 4 is recognized as World Animal Day, a tradition begun in 1931 by a gathering of ecologists in Italy, selected because it is the feast day of St. Francis of Assisi, the patron saint of animals. World Animal Day is celebrated around the world, but a smaller number of people celebrate World Animal Month, dedicated to recognizing that humanity must change in its attitude toward animals to ensure an equitable and ongoing relationship with the animals that share our world. Here at Eyes.com we are doing our part by celebrating the incredible diversity of visual systems in the animal kingdom.

One of the remarkable characteristics of many animal eyes is that they have multifocal lenses. Multifocal lenses are not uniform in focus like the natural human lens, but is more similar to those utilized by some intraocular lenses like ReZoom and ReSTOR. Multifocal lenses focus light from different distances and of different colors on the retina. This is a very good solution for allowing animals to have superior distance, close-up, and color vision outside the limits of accommodation of the lens.

For some animals, distance vision is very important, such as ruminants adapted to living on the plains, such as antelopes. Being able to focus on distant objects allows antelopes to better spot predators approaching, and multifocal lenses improve this ability with zones dedicated to near and far vision, similar to the ReZoom. For other animals, color vision is the key advantage of multifocal lenses. The cornea and lens of our eye have a prismatic effect: they bend light of different colors at different degrees, making a different focal length for lights of different colors, so seeing a clear color image can be difficult. In a relatively large eye, like the human eye, there is enough depth of focus that the different focal lengths can be overcome and we can see clearly in color without special adaptation. But in small eyes the poorly focused light of different colors creates too much blur, preventing a clear image. Multifocal lenses overcome this problem by having zones dedicated to bending each color of light differently so that they all focus on the retina.

One problem with multifocal lenses is that under bright light conditions, the pupil contracts and may shut out some of the refractive zones. The designers of the ReZoom lens overcame this problem by having zones dedicated to near vision and distance vision in both the outer and inner portion of the lens. Most animals, however, overcome this problem with slit-type pupils. When these pupils contract, they reduce the amount of light entering the eye, but ensure that the light entering the eye still passes through all the necessary refractive zones.

Some animals, though, have multifocal optical systems with round pupils, most notably the common house mouse. The house mouse has a rare visual system controlled by a “switching” pupil that remains fully open in even bright light conditions, then contracts quickly from to being almost fully closed in a very narrow range of light conditions. This controls the specific focal zones that light passes through, preventing blur and helping the mouse have quality vision under all light conditions.

Looking at the visual systems of other animals reminds us how complicated visual systems are and all the factors that have to be balanced to ensure you have clear vision under a number of light and use conditions. When you are considering refractive surgery, cataract surgery, or other procedures that alter your visual system, your ophthalmologist should talk to you about achieving the best possible balance for your eyes and how you use them.