fix and fit

Lens Curve Basics

by Alex Yoho, ABOM

Knowing which base curve a lens will be ground on is key to successful dispensing. The majority of frames available today, and certainly in the recent past, are shaped to a +6.00 diopter curve.

The intent is that these frames can be used without modification for an average prescription range. They also can be curved slightly to accept a +8.00 base lens, or, flattened slightly to accept a +4.00 base lens.

Generally this holds true with basic frames, but there are some recent frame designs that do not allow the traditional modification of using curving pliers to flatten or increase the eyewire curvature.

Examples include frames made with flat metal fronts and eyewires welded in back or metal frames that have the appearance of plastic frames, but with separations and eyewire screws. Both of these designs are very unforgiving when it comes to adapting to the lens curve.

BALANCING ACT

Several things need to be balanced when designing eyewear for patients. A frame shape should complement the patient's face, but also keep in mind the prescription and the effect of the frame size and shape to achieve the best overall balance of cosmetics and function.

■ +8.00 BASE: If the lens will be ground on a +8.00 base curve, for example, the bevel will likely follow the base curve of the lens.

If the patient prefers a rectangular frame shape, guide him or her to a frame that leans toward a wrapped look. Also, look for a frame size that keeps the eyes centered horizontally and one in which a rectangular shape cuts into the thicker part of the lens on top and bottom, thus accentuating the thickness. As opposed to a rectangular frame choice, a rounder shape would not have this problem.

■ FLATTER CURVES: If it is determined that the lenses will be made in a flatter base curve, such as a +4.00 or +2.00, consider a flatter frame, as the bevel of the lens follows the front curvature of the lens.

The frame will cut into the thinner portion of a minus lens, so this aspect of the shape is not a limiting factor. However, the longer a shape is horizontally, the thicker the lens gets, particularly if the eyes are not centered well within the shape.

Finding that balance in design parameters is the most important thing to do for the patient.

Understanding the curves involved will help create beautiful eyewear, reduce poor lens retention, and thus, patient frustration, and make your lab hold you up as an example of a dispenser who really knows his or her stuff. EB