LINE OF SIGHT:
Progressive-focus eyeglasses have been around for 45 years. But in July a German optical company introduced a new lens with a larger field of view to the United States. Eye experts explain the new science.
|In Carl Zeiss Optical’s new Gradal Individual lenses, the mushroom-shaped viewing channel through which objects may be seen is wide at the top, where distance vision occurs. The lenses are narrow in the middle, where objects about 2 to 12 feet away should be in focus, and broader toward the bottom for reading and other close work. The channel requires a continuous increase in lens curvature, which also creates some blurriness and distortion on the sides. Eventually, the brain can compensate for focusing problems, but sometimes that requires patience. Opticians say that a small percentage of people who try progressives reject them.||Areas on the sides of the viewing channel are always out of focus. However, these regions are smaller in the new Zeiss lenses, and most wearers adapt, learning to point their head toward what they’re looking at.|
|Progressive lenses are thin—as little as .039 inch in the center—but tough: Ground from plastic and half the weight of glass, they must not shatter when a marble-size ball of steel weighing 0.56 ounce is dropped on them from 50 inches up. The American National Standards Institute requires that lenses be ground within ± 0.12 diopter (a measurement of lens power) of the actual prescription.|
To hold down costs, manufacturers of regular progressive lenses pregrind blanks to various common curvatures on the front. They cut the backs using averages for such variables as frame tilt, distance between pupils, and distance between the lens and the cornea. By contrast, the new lenses from Zeiss are custom-made for each person. Zeiss uses multiple measurements from the wearer to drive computerized machines that grind and polish both sides of each lens to within 0.01 inch of the specifications. The new lenses cost $450 to $600 a pair, about 20 percent more than regular progressives.
|Each of Zeiss’s new lenses has three coatings: a polysiloxane hardening lacquer to reduce scratching; six to eight alternating layers of metal oxide and quartz, which cut down on reflection by interfering with certain wavelengths of light; and a proprietary coating to reduce adhesion of water, dust, and grease. Plastic lenses absorb virtually all ultraviolet radiation and do not need UV coatings.||SOLA International introduced its own custom-made progressives in the United States in February to compete with Zeiss. Hoya Corporation and Essilor of America are expected to follow suit. “The time is coming when the flat-top bifocal will be something for the Smithsonian,” says ophthalmologist Robert Campbell, editor of Refractive Eyecare for Ophthalmologists, a national journal. “These progressive lenses just keep getting better.”|