The first progressive lenses were made by putting the patient’s near prescription on the front of the lens and the patient's distance prescription on the back of the lens. Today lenses are even more advanced.
This was done by creating a mould or a form with the specific shape required to achieve one of the common reading prescriptions, and then pouring a clear plastic resin into the form. Once hardened, the lens blank can be removed from the form and set aside until needed.
When someone required a progressive lens with that reading prescription on the front of the lens, that lens blank could be selected, and the back surface could be shaped in a way that put their exact distance prescription slightly above the reading area.
There were certain compromises in this process, that our industry accepted in order to get rid of the bifocal line and to make progressive lenses consistently and repeatedly the same in their design. It worked, but these compromises resulted in differences in the magnification effects that patients experienced as their eyes moved from the top to the bottom of the lenses and back again. These magnification differences are what cause the “swim and sway” and other “distortions” patients experience in some progressive lens designs. About 80% of bifocal lens wearers could adapt to these magnification differences and successfully wear their progressive lenses. Roughly 20% could not, and went back to their lined bifocals.
In our day we use a completely different manufacturing process that takes advantage of ultra-precise 3-dimensional lathing techniques to make an entirely custom lens for each and every patient. We use computers to make calculations and determine how much magnification needs to be added or subtracted from the top or the bottom of the lens in order to minimize the distortions that make progressive lenses easier to wear. The computer tells us what thickness and shape needs to be used in every position, all over the surface of the lens in order to achieve that magnification profile. Then the three dimensional lathe creates that exact lens design by shaping the front, the back, or both. We no longer use a mould or a form in the fabrication process. That’s why these lenses are called “Free Form Lenses”.
Apart from all the advantages we’ve just described for basic progressive lens designs, Free Form Lens technology also provides the flexibility necessary to make individual adjustments for the patients frame. Have you noticed that frame styles and fashions have become much smaller since 2009 or 2010? Prior to that time, when conventional progressive lens design was all we had, we needed a minimum distance separating the distance from the reading prescription in the patients eyeglass frame. Therefore, frame styles were deeper and larger. Now with Free Form Lens technology, we can specify a specific lens design suitable to the patient’s frame depth. Now frame choice can be much more varied in fashion and style.
There are other advantages of Free Form Lens design that relate to the degree of customization they afford the optometrist. Variable corridor length, variable inset, the ability to specify frame wrap angle, pantoscopic tilt, and vertex distance, give us the opportunity to consider the patients down-gaze angle, working distances, and frame choice in exponentially more specific ways than we could before. All of these possibilities afford us the opportunity to make progressive lenses that work for patients.
If you are a progressive lens wearer in need of a prescription update, we would love to put our passion for progressive lenses to work for you. Your next pair of progressive lenses will work best for you if we begin with an accurate eyeglass prescription. So please call and book an appointment for your next eye exam at Doig Optometry.