Progressive lenses, or those which allow the wearer to shift from distance to near vision, entered the eyewear market decades, ago but their design is finally starting to improve.
Progressive lenses made their commercial debut in the middle of the last century. Also known as progressive addition lenses (PAL), the lenses enable eyeglass wearers to transition from distance to near vision without the image jumping when the eyes shift from one distance zone to another. They also have a cosmetic benefit: there is no line as there is with bifocals.
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In the past, producing these lenses was no easy feat. The traditional process required a semi-finished lens with a standard front. The prescription would be ground onto the back surface with a generator. Calculating the geometry to impart on the lens involves complicated mathematics. "You can imagine doing this without Excel spreadsheets to do all of your calculations," said Dennis Fong, OD, clinical instructor at the UC Berkeley School of Optometry. "Everything was originally done by hand and then you see pictures of these old generators that made forms and then you use the forms to create the complex curvature."
Although the process used to create the lens has steadily improved over years, drawbacks remained, including the fact that distortion or aberration is present in the lower periphery of the lens. "That type of distortion bothers people a lot," Fong said. "It can make them feel nauseated. And the higher your prescription, the bigger the difference between the top and the bottom, the more distortion is going to be out there. Those were the negatives in general of the traditional progressive design."