Aspheric Advantage
Improved optics, better visual acuity, minimal distortion, slimmer, flatter, thinner, lightweight. 
More patients should be in aspheric design: Here's why
By Karlen Cole McLean, ABOC, NCLC

Perhaps the most powerful truism about asphericity is that vision through aspheric lenses is closer to natural vision. Aspheric design allows flatter base curves to be used without compromising optical performance. The basic difference between a spherical and an aspheric lens is that a spherical lens has one curvature and is shaped like a basketball. An aspheric lens curves gradually, like the football below.

Aspheric curves are used in several different ways but for similar reasons. They control three key undesirable elements in a lens: Spherical aberration, distortion, and marginal astigmatism.

Spherical aberration. When light enters the surface of a spherical lens, it results in a range of focal points of incoming light. This can cause blurry vision and peripheral distortion. With an aspheric lens, the surface curvature has less of a severe angle, so there is less power gain at the edges. This creates a single focal point and allows clearer, sharper vision and reduced peripheral distortion.

Distortion. Distortion occurs when straight lines are bent as they are viewed through the lens, due to increasing magnification or minification. Asphericity decreases power at the edge, which reduces or eliminates pincushion (plus lenses) or barrel (minus lenses) distortion.

Marginal astigmatism. An undesirable sphere and cylinder power gain when looking away from the optical center, marginal astigmatism can cause an astigmatic focus of light at the lens edge. Aspheric design reduces marginal astigmatism by reducing power gain at the periphery.

THE MARKET

Aspherics should be your first consideration with the majority of patients--for both optical advantages and cosmetic reasons. Aspheric lenses are the lens equivalent of a flattering frame.

"Aspheric lens designs flatter my customers with thinner, flatter lenses for a more natural look and improved optics, especially for hyperopes," says Judy Canty of Powell Focus Canty Opticians in Alexandria, Va.

The key to effective aspheric lens dispensing and getting patients to buy into the aspheric concept is effective demonstration.

"My best tool in describing the benefits of aspherics is my lens center. Rather than having to translate technical data to everyday language, I can show my customers the benefits," says Canty. "Being able to speak about the latest technology maintains a positive image about the product and our practice."

Clayton Eye Center in Atlanta now presents aspheric lenses in single-vision polycarbonate for children."We feature them as a high-end, specialty product," explains Kirk Smick, OD.

"We like the fact that the vision is sharper, the magnification/ minification is decreased, and the cosmetic appeal is better. The entire process begins in the exam room with the doctor mentioning these new products."

SELECTION

Au Courant Opticians, with several locations in Michigan, Florida, and New Jersey, relies on the "best lens for the patient" philosophy, and counts on the experience of their opticians to know when aspherics are appropriate.

"We go with aspheric lenses when the Rx, frame, and patient profile call for it," explains Nancy DiCosmo, AuCourant's president. "If the patient is wearing aspheric lenses in one frame, then they have aspheric lenses in all of their frames."

Jim Organ, president of Optical Fashion Centers, with seven stores in Ohio, says that 95 percent of the chain's business is in poly--most of it aspheric. "We also do aspheric PALs and emphasize AR. We're not afraid to mention a second pair."

He stresses the need to keep the aspheric message simple. "If patients spend on eyewear like they do on shoes, then with some technical information and focus on overall fashion and function value, sales will increase."

WHAT'S OUT THERE

PAL aspherics. Aspheric curves, which change gradually, are the basic ingredient in what makes PALs possible. Today's freeform PALs rely on asphericity for superior optical performance and have become the standard for PALs due to aspheric design. Each manufacturer's design is unique. Some apply backside aspheric curves, some frontside, and some both to achieve their patented PAL designs.

Corrected curve designs change base curves for optimal distortionless vision. Today, all lenses are corrected curve or "best form" lenses. Free-form lenses are the best lens form for a given Rx; the base curve provides the best balance of optics and
cosmetics.

Plus aspherics. Front curves of plus aspheric lenses gradually flatten toward the edge of the lens. As the lenses flatten, the back surface is brought closer to the lens' front.

The advantages: 1. Aspheric plus lenses reduce magnification for a more natural appearance, and 2. The decreased center thickness uses less material, resulting in less weight.

Traditional lenses in high-plus powers cause the wearer to look directly in the lens center for good vision. High-plus aspheric lenses minimize off-center errors, so patients can see without turning their heads. Plus lens distortion includes magnification, which is greater at the edge of a plus lens. This inward curvature is known as the pincushion effect.

Minus aspherics. Front curves of a minus aspheric lens get steeper toward the edge of the lens. Steeper curves at the edge thin the edges of the lens, which reduces lens weight and gives a more natural appearance.

Combine asphericity with high-index, and you have the best lens to reduce edge thickness. While the benefits of aspheric flattening are more apparent in plus lenses than minus, both lenses' aspheric properties feature less bulge. Minus lens distortion includes minification, which is greater at the edge of a minus lens. This outward bulging is known as barrel distortion.

Aspherics for low powers. While not as dramatic, asphericity in lower powers provides good vision when looking away from the optical center. Patients with low powers will benefit from better all-around-the-lens vision.

ASPHERIC ADVICE

There are landmines in the field of selling aspherics to be aware of. Lens clocks, for example, don't give true readings of aspheric surfaces. To identify an aspheric lens, it's best to visually inspect the lenses for a flatter appearance.

Some aspheric designs, however, are flatter than others. Many lens manufacturers are now identifying their aspheric products with laser engravings.

Check with the manufacturer for their aspheric brand identifiers.

Also remember that aspheric lenses can't be decentered to reduce prism. Prescribed prism must be ground into the lens by the lab.

The benefits of asphericity can be edged away, explains Chris Gregg, optician/owner of Inver Grove Heights Family Eye Clinic, located in Inver Grove Heights, Minn.

"You can cut off the benefits of an aspheric lens in a small frame," he says. "It's crucial that you use manufacturer cut-out charts, since that's the only way to determine if an aspheric lens is worth it for you and the patient."

Because aspheric lenses are flatter, they may cause "lash crash" on the backside of the lens, where the patient's lashes brush against the lens surface.

A concave curve on the backside helps position the lens further from the eye and provides a wider field of vision.

Finally, some sensitive patients may notice that things look different with their new aspheric lenses. This is because aspherics can reduce magnification or minification up to 20 percent, so objects will appear different in size.

Assure them that this is a positive change--their eyes will ease and enjoy the new view in just a few days.