Understanding Free-Form Progressive Lenses

Nearly everyone over the age of 45 is a presbyope (person that has started to need some help reading the dinner menu over the past few years) and has heard the stories about Progressive Addition Lenses (PALs); “I have no room to see the computer,” “There is too much distortion on the sides,” “I can’t play golf in these,” and so on…

Currently there is no perfect solution for Presbyopes (nope, not even LASIK) when it comes to eyewear. There are some contact lenses on the market that do a very good job of correcting distance, intermediate and near vision, but the percentage of people able (or willing) to wear contacts makes this a small number of people needing corrective eyewear. Eyeglasses remain the most popular and effective means of correcting the multi-faceted refractive errors that the presbyopic patient experiences, with lined bifocals and trifocals fading into obscurity as the “no line” progressive lens dominates that segment of patients.

Before we get into the nuances of how a PAL corrects vision, it is vital that we understand how prescription lenses work. In its simplest form, a prescription lens uses a combination of curves to correct the visual imperfection in the eye. For a hyperope (someone that is farsighted) the lens will have a more curved front and a flatter back curve, creating magnification. For a myope (someone that is nearsighted) the lens will have a flatter front curve and a more curved back, causing minification. No one can be a hyperope and a myope at the same time, not in the same eye at least, but many people can have astigmatism in addition to farsightedness or nearsightedness (or absent both of those), this is corrected by a second curve in the back of the lens (the top of this curve and the original curve meet at the same point but the ridge of the second curve is at a specific angle, like a football), causing the image to bend along that axis as it passes through the lens.

Now those are all single vision lenses and do not take into account the special needs of a presbyope, increased magnification for distances closer than six feet. Due to the crystalline lens within the eye stiffening over time, presbyopes are not able to adjust their own vision for near activities and need the assistance of more custom lenses. As we noted in the lenses for a hyperope, a more curved front increases magnification in a lens. Knowing this, multifocal lenses (bi, tri and PAL) increase the front curve of a lens in very specific areas. Older lens designs, the bifocal and trifocal, accomplish this by increasing the thickness of the lens with a noticeable line marking where that increase occurs. In a bifocal there is one line that shows the difference in thickness between the reading and distance, in a trifocal there are lines between the distance and computer, and computer and reading.

So, how does a progressive lens manage to correct for distance, computer and reading without those lines and why is there distortion on the sides that create limited computer and reading areas?

Nothing in the optical world has changed so much that we have been able to get away from using curves to correct vision (nothing will until we are able to makes lenses of varying density in an affordable way), so PALs still rely on steeper frontside curves to add magnification in the intermediate and near areas of eyeglass lenses, but technology has made it so that we can “blend” those lines away and “smooth” that thickness into less useable areas of the lens, such as the sides. By doing this (smoothing of the lens), optical lenses have been able to create a more natural feeling experience when moving through the various distances that people generally use, while still correcting a person’s vision. The increase in thickness is still present, but the lines have been eliminated and the increases so gradual that you probably cannot feel them if you rub your fingers across the front side of the lens. Using specific molds, optical lens manufacturers are able to make traditional progressives that have a uniform frontside, ensuring steep curves are present, and then they carve the specific prescription into the back of the lens for each patient as needed. This advancement in technology allows for more usable lenses that are cosmetically more appealing as well, but we still have that pesky distortion on the sides.

Traditional lenses cannot get away from having that distortion because they all use a limited number of molds for the front surface of the lens. Meaning that only the back of the lens is customized for the specific prescription and even then it is only customized using one or two curves. This way of making optical lenses does not allow for the thousands of curves (curves that would have to be imperceptible to the human eye and not allow for dust, water or light to settle on them) that would be needed in order for a patient to see with minimal distortion through all parts of their eyeglass lens. So, that distortion is simply a fact of life for presbyopes using traditional PALs.

Thankfully we are in the era of “free-form” digital lenses; PAL lenses that are custom formed on both the front and backside, not using a limited number of curves to produce the prescription, but using sometimes up to half a million points of reference in order to create the most natural and usable area for the presbyopic wearer. By customizing the lenses with so many specific points in mind, these lenses are able to create wider computer and reading areas while ensuring the transition from one area to another is still so smooth most people do not realize it. These lenses use the latest technology to create an experience that brings together viewable area, lens integrity and wearer comfort. Until we are able to produce lenses with variable density materials (another key factor in optical lenses) a digital free-form PAL is the best optical experience out there for presbyopes.

So for now, the realm of fully customizable, digital free-form lenses (also available in a single vision variation) is where we are at and the ability to see clearly in the distance, intermediate and near areas is the best it has ever been within a single pair of glasses. For anyone that found a progressive lens to be a challenge or have limitations, the solution may be found within.

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