Multifocal Designs
Other Multifocal Styles

The Franklin Style bifocal, also commonly known as the Executive bifocal, is designed so the reading portion spans the entire diameter of the lens. The optical center of the reading portion is typically located on or near the transition line (which minimizes image jump) and may either be coincident with the geometric center of the lens or slightly decentered in. This style results in somewhat thicker, heavier lenses, so care should be taken not to use it in higher powered prescriptions. As a result, base curve availability is limited.

The Ultex bifocal is rarely used today. Although it offers a potentially large reading area, the optical center of the reading portion is positioned at the bottom of the segment about 18 mm from the transition line which can result in excessive amounts of image jump in higher powered reading additions. This style has largely been eclipsed by improved multifocal designs.

The Blended bifocal is a round segment, ranging in size from 22-28 mm diameter in which the transition from the carrier lens to the segment has been smoothed making it difficult to see. Although this style is cosmetically appealing for the patient who prefers a lens with no obvious line, the blended transition zone usually consisting of 2-3 mm of unusable area, can sometimes be problematic. Also there can be considerable image jump in larger segment sizes.

The Progressive bifocal has achieved tremendous popularity in recent years with virtually every major lens manufacturer now producing their own version of this multifocal style. A progressive lens contains no transition line and no image jump. Instead, there is a gradual or progressive change of power along a �channel� from distance to near. The power of the lens in about the middle of the transition area is approximately equal to the intermediate prescription. The cosmetic advantages of this lens, the smooth transition from distance to near with no image jump, combined with a usable intermediate area have contributed to the popularity of this style. However, the lens is not for every patient. On either side of the channel lies unusable areas of distortion which can be difficult to adapt to. The intermediate zone is relatively small, rendering it impractical for the advanced presbyope who depends on a wide field of vision through the intermediate area. The ideal candidate for this lens is the early presbyope since lower add powers result in wider intermediate and near zones and less peripheral distortion. Accurate monocular P.D. and �seg height� measurements are recommeded for successful fitting.

The two trifocal styles shown on the previous page are a franklin style lens and a flat top. If a 28 mm flat top lens also contains an intermediate zone which is 7 mm deep, it is called a 7 x 28 flat top trifocal. Some common flat top trifocal dimensions include: 7 x 25; 7 x 28; 7 x 35; 8 x 35. The intermediate zone of the franklin style trifocal is typically 7-8 mm deep and has fitting limitations similar to the bifocal version of this style. The power of the intermediate zone is most commonly equal to 50% of the add power. Although 40%, 60%, and even 70% intermediate zones are occassionally prescribed, they usually need to be specially ordered.