The “best form” of a lens is one that provides the same vision at center and in the periphery, as the eye moves normally behind the lens. For finished lenses, manufacturers choose both front and back curves; for semi-finished lenses, a recommended series of front curves are provided usually in a base curve chart. When you order lenses from the lab, they already have these charts loaded in their computer lab management systems so the correct base curve is chosen for your order. For the best vision in a lens, it’s better to let the lab choose the base curve. In best form lenses, the front surface curve is spherical i.e., the same curve or radius in all directions from center.
In contemporary frames, we have new challenges. First, frames tend to be “flatter;” in larger ones, lenses often bulge out the front. To make lenses bulge less and fit today’s flatter frames consider aspheric lenses.
Ashperic lenses (Fig. 2) that have been flattened to look better and see better, too. They have front surfaces that are “not-spherical;” they flatten towards the periphery in plus prescriptions and steepen towards the periphery in minus. This change of curvature is calculated to correct for the peripheral blur that would be created when a flatter base curve is used that is not best form. Benefits for the wearer are the lens looks better in the frame and is less magnifying. Less magnification makes both the wearer’s eyes and the world that they see more natural in size. This is one reason why patients tell us that they see better with their aspheric lenses than their previous glasses.
I like to use aspheric lenses for young hyperopes. Parents are always concerned about the cosmetics of plus lenses on their children. After all, no parent wants their child getting teased because they are wearing thick, magnifying lenses. By using flatter, thinner polycarbonate lenses with an anti-reflective coating, those +4.00 and +5.00 diopter prescriptions can actually look like half the power. In addition, they become patients for life. Parents return year after year as their child grows up because I understand and meet their needs and concerns.
Jim Mitchem
ABO Dispensing Optician
Allina Medical Clinic - Cottage Grove, MN
What Would You Do?
PATIENT SITUATIONMrs. Jefferson, a new patient wants desperately to make her new glasses look better than the ones she’s wearing now. She was on a strict budget last time so they were plastic lenses and frames, no AR. She’s chosen a semi-rimless that is the same 52 eyesize, Rx is +3.50 sphere. What can be done?
SOLUTIONFirst, Mrs. Jefferson’s lenses are thicker in the center than the edges and bulge out from the front of the frame. Her eyes are also quite magnified.
Since she wants to keep the same frames size, increasing the index of the lens material will reduce the lens’ center thickness. The edge thickness will be the same (be sure that at the thinnest edge, the thickness is a minimum. 1.8mm is a good standard for semi-rimless).
Recommend 1.67 high index and lenses that are aspheric. The index reduces thickness by up to 40%, the flattened aspheric design further reduces thickness and lenses will not bulge out the front of the frame. The flattened lens design also reduces the magnification effects so the world appears more natural in shape and size while Mrs. Jefferson’s eyes look better too.
Talk to your lab about making a standard vs. aspheric lens demonstrator. Then use it to show the differences to patients so they better understand and can see how much better apsheric lenses look.