Feb
2004

Lens Choices

Lenses courtesy of SOLA Technologies photographed by NEDJELJKO MATURA

Ten years ago, many dispensers considered aspherics as exotic specialty lenses.

A lot has changed since then. Today, most dispensers are at least familiar with aspherics and the visual and cosmetic benefits these lenses offer. Specifically, asphericity thins the lens, widens visual fields and improves cosmetics by reducing magnification in plus prescriptions and so-called “minification” in minus prescriptions.

Because aspherics are a prescription-driven product, dispensers need to identify appropriate patients and effectively present this high-tech option to them. Experienced dispensers stress aspherics to patients ranging from -4.00 D and higher for minus and +2.50 D or higher for plus.

The distinctive cosmetic improvement of thinner edges for minus and flatter centers for plus are immediately more apparent to patients than the improvements in acuity and reductions in peripheral distortions.

“We use aspheric technology whenever we need an especially thin, flat profile or in higher lens powers where the patient may encounter aberrations,” says Cindy Elkin, optician, co-owner, Point of View Eyewear in Falls Church, Va.
“For the most part, we use asphericity for its excellent cosmetic benefits.

“Because aspheric lenses are flatter—and therefore thinner—than conventional lenses, they are an excellent choice when the lightest weight eyewear is the goal,” Elkin adds. “Because they decrease magnification of the eye behind the lens, they are a great cosmetic benefit for hyperopes in higher-plus powers.”

Dianna Finiscey, optician and president of Wagner Opticians in Washington, D.C. often recommends aspherics for patients with prescriptions in the +3.00 to +4.00 D range. “It depends on the frame size, pupillary distance, the patient and the overall power of the Rx,” she says. “The larger the frame, the smaller the PD, the higher the plus power—all are taken into consideration.”

Richard Rubin, optician/manager of the The Eye Center, Pembroke Pines, Fla., favors aspherics for high-minus prescriptions. “Recently, we put someone who is a -7.00 D into 1.67-index aspheric lenses,” says Rubin. “It had a real wow factor, not only because of the thin appearance and light weight, but because the vision was improved.” Rubin adds his patients are finding that vision in 1.67 aspheric high-index lenses is better, not just peripherally, but overall throughout the lens.

Many dispensers say that children are often good candidates for aspherics. “Because a thinner, flatter lens is lighter and more comfortable to wear, aspheric lenses are a good choice for children,” notes Elkin. “Lighter weight means that the frames will be less likely to slip down.”

Aspheric lenses are important when you have high-plus powers in tiny frames, points out Finiscey. “With less magnification and better mounting into the frames, the eyewear becomes much more user-friendly and appeals to kids who may have an aversion to glasses and to parents who want good-looking glasses for their kids.”

Seniors can also benefit from aspherics, particularly when lens comfort is a concern. “A thinner, lighter-weight lens is less likely to irritate the thinner skin of older people,” observes Elkin.

Many patients, especially those with high-powered prescriptions, undergo an adaptation period with aspheric lenses. Although any discomfort is generally minimal and adaptation is usually brief, wearers should be informed their eyes must adjust to asphericity. If the dispenser takes the time to explain lens design, the transition from a spherical design is smooth and satisfaction remains high.

Dispensers also report that non-adapts are practically non-existent, but precise fitting is critical. If the patient is not looking through the optical center of an aspheric lens, they will be viewing through an off-power curve. To ensure a wearer’s ease of adaptation, dispensers emphasize taking monocular PDs when fitting aspherics.

When fitting aspherics, the general rule is to place the OC of the lens slightly below the center (approximately 2 to 5 millimeters) of the pupil, or, when observing the pantascopic tilt of the frame, drop the OC 1 millimeter per approximately 2 degrees of pantascopic tilt to best align the visual and optical axis. In general, the pantascopic tilt is steeper for plus lenses than minus lenses.

Most importantly, all aspheric designs differ, so dispensers should check with lens manufacturers for fitting specifications for each brand.
What is Asphericity?
Simply put, aspheric lenses feature a different lens design than spherical lenses. Yet asphericity is far from simple. To understand the concept, consider that there are two components of a lens that affect how light travels through the lens: the index of refraction and the lens curve. The index of refraction is a function of material, but the lens curve is a function of design. By manipulating the curves on the front and back surface of a lens, a prescription power is created.

For some prescription powers, spherical lenses require the selection of steep base curves to compensate for the angle at which the eye looks through the lens as it moves from the center to the periphery. The result is thick edges for minus lenses and thick, bulbous centers for plus lenses. When a flatter base curve is selected, some aberrations increase, particularly oblique astigmatism and off-axis power errors. These distortions are inherent in the design nature of spherical curves, which are a continuous shape and power.

By contrast, aspherical base curves change shape and power across the surface of a lens. That change in power is gradual. And because of this change in power, the lens is flatter but the peripheral distortions are minimized and marginal astigmatism and off-power axis error are not created.

Aspheric designs require computer programs to calculate the numerous power or curve changes.  Asphericity is defined by a polynomial function, which is a mathematical expression using variables and coefficients.

Why are there are so many different aspheric designs? Each manufacturer uses different coefficients in that polynomial, creating different design characteristics. Some designs feature more asphericity than other designs, which means that the lens curve has more changes in power. In general, the higher the prescription, the more asphericity needed to flatten the lens and minimize distortions. Although each design is unique, the optics and cosmetics are superior to spherical lenses.

 

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