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How contact lenses work to correct vision is the same way eyeglasses do: they alter the direction of light rays to focus light properly onto the retina.

If you are nearsighted, light rays focus too early within your eye — they form a focus point in front of the retina instead of directly on it. Contact lenses and eyeglasses correct nearsightedness by diverging light rays, which reduces the eye's focusing power. This moves the eye's focus point backward, onto the retina where it belongs.

If you are farsighted, your eye does not have adequate focusing power — light rays fail to form a focus point by the time they reach the retina. Contact lenses and glasses correct farsightedness by converging light rays, which increases the eye's focusing power. This moves the eye's focus point forward, onto the retina.

Contact lens and eyeglass lens powers are expressed in diopters (D). Lens powers that correct nearsightedness start with a minus sign (–), and lens powers that correct farsightedness start with a plus sign (+).

So why are contact lens so much thinner than eyeglass lenses?

In large part, it's because contact lenses rest directly on the eye, instead of roughly a half-inch (12 millimeters) away from the eye's surface like eyeglass lenses.

Because of their proximity to the eye, the optic zone of contact lenses (the central part of the lenses that contains the corrective power) can be made much smaller than the optic zone of eyeglass lenses.

In fact, the optic zone of eyeglass lenses is the entire lens surface. The optic zone of contact lenses is only a portion of the lens, which is surrounded by peripheral fitting curves that do not affect vision.

It's something like looking out a small window in your house: If you are standing very close to the window, you have a large, unobstructed view of the outdoors. But if you are standing across the room from the window, your view outside is very limited — unless you have a much larger window.

Because contact lenses rest directly on the cornea, their optic zone only needs to be roughly the same diameter as the pupil of the eye in low-light conditions (about 9 millimeters). In comparison, to provide an adequate field of view, most eyeglass lenses are greater than 46 mm in diameter. This larger size makes eyeglass lenses much thicker than contact lenses.

Watch this video on what causes blurry vision and how we can correct it.

Also, eyeglass lenses must be made much thicker than contact lenses to keep them from breaking upon impact. Lenses for nearsightedness in eyeglasses must have a minimum center thickness of 1.0 mm or greater to meet impact resistance guidelines.

Contact lenses can be made much thinner. In fact, most soft contact lenses for nearsightedness have a center thickness that is less than 0.1 mm.

So it's the combination of significant differences in wearing position, optic zone diameter and minimal thickness to ensure structural integrity that makes contact lenses much, much thinner than eyeglass lenses of the same power.

About the Author: Gary Heiting, OD, is senior editor of AllAboutVision.com. Dr. Heiting has more than 25 years of experience as an eye care provider, health educator and consultant to the eyewear industry. His special interests include contact lenses, nutrition and preventive vision care.

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Like This Page? Please Share! How contact lenses work to correct vision is the same way eyeglasses do: they alter the direction of light rays to focus light properly onto the retina. If you are nearsighted, light rays focus too early within your eye - they form a focus point in front of the retina instead of directly on it.

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animal vision 2

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While humans have adapted to eye problems by means of glasses and eye care, we aren't the only ones that have succeeded in developing interesting ways to beat natural selection. Nature is full of interesting creatures that flourished in their habitats due to some sort of visual prowess. However, for every creature that has succeeded because of their eyes, there are also many that have failed because of a lack of eyesight.

The world is full of harsh terrains and the desert may well be the harshest. With so many potential eye threats present like sand and constant sunlight, it would be helpful to have a pair goggles. Animals don't exactly have access to goggles but there are some mammals, such as camels, that have a solid substitution. Camels have an extra protective eyelid that acts as a nictitating membrane, covering up both their eyes in order to easily maneuver through the unrelenting wastelands of the desert. This third eyelid, as it is often referred to, helps stop the camel from getting debris stuck within its eyes and offers some sun protection. However, this third eyelid isn't only meant for the heat.

On the coldest parts of earth, another animal uses it for a different purpose. Polar bears living on the north and south poles have a similar nictitating membrane that they use for several different reasons. The brutally cold winds of the North Pole and constant swimming through salt water can cause extreme irritation to polar bears, but thanks to the nictitating membrane they are safe from these potential harms. Both of these animals have thrived in extreme temperatures for thousands of years.

Snakes dominate in these areas because of their special eyes equipped with infrared (IR) vision.

Sometimes, visual prowess isn't all about protection and defense. In the case of several predators within the animal kingdom, it's all about the offensive. Most animals use their eyes when it comes to stalking and hunting prey, however there are some that take it a few steps further. There comes a point where regular eyes are no longer enough, especially in places like jungles and forests where it's easier for smaller prey to camouflage. While many hunters might be left with a harder time finding food, there are some whose visual advantage helps them slither their way to the top of the food chain. Snakes dominate in these areas because of their special eyes equipped with infrared (IR) vision. IR vision allows animals to see other animals within a spectrum of body heat. Because of this, animals that hide by means of camouflage are left completely visible and vulnerable. Snakes then have a much easier time catching these animals. As easy as it may already seem for these snakes to hunt, the true power of infrared vision is witnessed at night when everything is pitch black and other animals can no longer see as well as in the daylight. On top being able to see prey more easily, one of the most impressive aspects of infrared hunting is that the snakes can pinpoint the best spot to bite its prey by paying attention to where its organs are, resulting in a clean kill.

Animals come with a wide range of hidden tricks for surviving out in the wild. In some cases the animal may use its eyes as a weapon. In the same way that some humans are afraid of blood, so are some of the members of the animal kingdom. The horned lizard isn't a very fast or strong reptile, but it does manage to get by thanks to its amazing eyes. Whenever a bigger animal is coming to attack, the horned lizard fires off a shot of blood from its eyes in order to subdue it before it can do any damage. The lizard then hurries off as the predator is blinded by its thick blood.

Nature is a great reminder of how important our eyes are and why we need to take care of them. Like in nature, having a better set of eyes can give you a competitive advantage that will help us succeed.
Sources:

Davis Vision does not provide medical advice, diagnosis or treatment. More information is available at Site Content & Member Care.

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While humans have adapted to eye problems by means of glasses and eye care, we aren't the only ones that have succeeded in developing interesting ways to beat natural selection. Nature is full of interesting creatures that flourished in their habitats due to some sort of visual prowess.

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Newsela | Big Questions: How do glasses help us see?

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Glasses work by helping the eyeball to focus light onto a very exact spot, called the retina.

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Whether you’re nearsighted or farsighted, at some point we all need an opthamologist. Learn about the windows to your soul, how eyes work, and how to take care of them!

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Different animals have different kinds of color vision. Some have very poor color vision & others have very good color vision. Some have super color vision