March 9, 2013 | POSTED BY | Articles, Pre-Optometry School, Study Resources
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With the use of smartphones, computers, laptops, and tablets, information is seen in a variety of different ways. Technology has increased the potential knowledge one can have access to. It is quite a feat to be able to look down at our phones and get information about virtually anything out of thin air. From our desktop computer screens to our favorite touchscreen devices, we find ourselves focused on an illuminated screen for hours on end. While we take this ability for granted, we view the advances of technology as an improvement in our daily lives. As technology expands we are going to need to find new ways to adapt to receiving visual information. Adaptation to this technology comes at ease for people under the age of 40. As age progresses one tends to lose this ability to see up close, which will make it difficult for those older than 40 to receive this information from their screens.

To focus in a variety of distances is an incredible adaptation of the eye. The ability to see a target at a distance and then refocus to another point almost instantaneously is known as accommodation. Our eyes are at rest when viewing at a distance but with distances less than 20 feet the eye requires the help of the crystalline lens and the muscles surrounding the lens to get the image into focus. The muscles surrounding theses lenses actively contract and flex changing the shape of the lens to provide this ability to focus at an image clearly at close range. This shows how the eyes and the brain can work together to provide a great adaptive physiological ability.

At birth our eyes have about 20 diopter of focusing ability. Diopters are the unit of measure of an optical system. With this range of accommodation the eye could focus at about 5 cm (2 inches) away from the eye to create a clear image. By the age of 40 our ability to accommodate is reduced to about 6 diopters of accommodation. This decrease in diopters is caused by reduction of the elasticity of the crystalline lens, liquefaction of the vitreous humor, and the multilayering of the lens itself. This results in the eye beginning to have trouble comfortably seeing at near objects, a condition known as presbyopia.

As we age the use of the eyes starts to reach it’s limits. People who are reaching the age of 40 begin to have minor discomfort while performing basic actions. Those who are having trouble reading a book or the newspaper will tell you that their “arms are not long enough” when trying to read at near. Since This can be a troubling time for many people since this change in their vision can affect their daily activity. A person uses his or her eyes in different ways. Whether reading a book for leisure, working on fine detail on a piece of jewelry, or playing a very aggressive sport, each individual has a unique adaptation to their vision, and to lose this ability can affect their livelihood.

The solution to overcoming the inability to accommodate is to use a pair single vision glasses or to use bi-focal or tri-focal glasses. This allows the patient to have a slight magnification through their glasses to overcome the lack of accommodation their eyes need to see clearly up close. An optometrist prescribes this type of magnification or “add” power to their eyeglass prescription. This “add” will allow the eyes to remain at rest when performing near tasks. The problem with these types of corrective lenses are the finite focal point these lenses provide to create a clear image. Moving away from the focal range this will cause the image to become blurry. This also limits the field of view a person has when he wants to look at varying distances such as looking down to read and looking at a point in the distance.

These corrective lenses have not adapted well to the change of visual demands we face today. Our need to find a different approach for those who are experiencing presbyopia who are wanting near natural vision. This is where the use of progressive additional lenses, or PALs have proven to be a great solution to help aid those who require almost near natural vision. Marketed as “no line bifocals,” PALs are lenses that have a variety of focal points as it progresses into the patients full add power prescribed. Compared to bifocals and trifocals, these lenses lack the visible line that changes from their distance prescription to the patient’s add power. Instead of having this line, the magnification of the lenses is generated into the lens matrix while manufacturing the lens. This allows the radius to change and apply the add power as needed when the patient looks to read at close range.

When a patient has a pair of progressive lenses on they are given a variety of distance zones. There are three distance zones: the first is the distance zone where the patients’ distance Rx is, fitted when the eyes are looking straight ahead. The second is the intermediate zone where the add power is gradually added into a “channel” to allow for the person to see at  varying distances. This allows the patient the ability to seeing clearly at points that may not be at reading distances. Finally the near zone of the progressive is at the bottom of the channel where the patient has the prescribed full add power of their spectacle Rx. As they look down they are given that slight magnification through the lens to be able to see the items that were once unclear for them by their lack of accommodation.

The key to a successful fit for those who are wearing progressive lenses are a proper measurement of the patients’ monocular pupillary distance, proper segment height measurement, and a frame with a sufficient vertical depth. Each of these measurements are taken prior to the patient receiving his or her glasses and are necessary for the optical to evaluate what the person needs. Choosing the right frame is important because the choice of frames determines the channel of the progressive. The deeper the frames are the more room the progressive has to transition into these different focal ranges needed to see at varying distances. The fit of the segment height determines where the patient eyes will be when looking through the distance zone of the lenses. For the monocular pupillary distance this measurement is crucial for the orientation of how the add power will progress in the channel.

With the different focal points there is a slight adaption time for a person to get used to wearing progressives. The person has to understand that there are different focal ranges and they are required to find the “sweet spot” of the lenses by simply tilting their head up and down. This adaption time can range from a couple days from a couple of months depending on the persons’ daily use of the lenses and the comfort one has to the adjustment of varying magnification. Those who have adapted well to the progressive lenses will have no trouble changing from different focal points throughout the day.

With our world constantly changing and making us adapt to the visual information available, progressives have proven to be the best solution for those experiencing presbyopia. Being able to use these PAL lenses shows how incredible the brain and the eyes are willing to adapt to these visual changes. We can rest assured that age will not hinder our use of new technology with the help of progressive lenses.