Bifocal lenses provide a solution for a common vision change that occurs with age, known as presbyopia. Presbyopia is the gradual loss of the eye’s ability to focus on nearby objects, making tasks like reading small print increasingly difficult. Bifocals eliminate the need to constantly switch between separate pairs of glasses for distance viewing and reading. By combining two distinct optical powers within a single lens, they offer clear vision for both far-away objects and close-up tasks.
Decoding the Bifocal Prescription Notation
Understanding a bifocal prescription requires familiarity with several specialized abbreviations and the meaning of the numerical values provided. The initial part of the prescription corrects distance vision and uses terms like OD (Oculus Dexter) and OS (Oculus Sinister) to denote the right and left eye, respectively. The SPH (Sphere) value indicates the main lens power in diopters needed to correct nearsightedness (marked with a minus sign) or farsightedness (marked with a plus sign).
Astigmatism is corrected by the CYL (Cylinder) and AXIS values. The CYL column lists the additional power required, while the AXIS number, ranging from 1 to 180, specifies the orientation of this cylindrical correction on the lens. These values ensure sharp distance vision.
The defining element of a bifocal prescription is the “ADD,” or addition power, which is the extra magnifying strength needed for near vision tasks. This value is almost always positive, typically ranging from +0.50 to +3.50 diopters, and is usually the same for both eyes. The ADD power is mathematically combined with the SPH power to determine the total near vision prescription; for example, a distance SPH of -1.00 and an ADD of +2.00 results in a near vision correction of +1.00. This value dictates the magnification placed in the lower segment, allowing the wearer to focus comfortably on objects held about 16 to 18 inches away.
How Bifocal Lenses Are Physically Structured
Bifocal lenses are physically divided into two distinct optical zones. The larger upper portion contains the prescription for distance viewing, while the small segment in the lower half holds the additional power (ADD) necessary for near vision. A visible line separates these two powers, marking an optical discontinuity between the distance and near correction zones.
The shape of this near-vision segment, often called the “seg,” can vary. Common designs include:
- The Flat-Top (or D-Seg), which features a straight horizontal line.
- The Round-Seg, which has a half-circle shape.
- The Executive or Franklin style, which dedicates the entire lower half of the lens to near vision.
The placement of the segment is determined by a precise measurement called the segment height, ensuring the reading zone aligns correctly with the wearer’s typical line of sight. The near segment is positioned low because the eyes naturally look downward when focusing on close objects.
Adjusting to Wearing Bifocals
The transition to wearing bifocals often involves a brief adjustment period, as the visual experience differs significantly from single-vision lenses. One common initial difficulty is the phenomenon known as “image jump,” which occurs when the eye crosses the visible line separating the two lens powers. This abrupt change in magnification can cause the image to suddenly shift upward, leading to temporary disorientation and a feeling of unsteadiness.
To adapt successfully, wearers must train their eyes and head movements to look through the correct lens portion. For distance viewing, the head should remain level, utilizing the upper section, while reading requires dropping the gaze into the lower segment. When walking, particularly on stairs, it is important to tilt the chin down slightly to view the steps through the distance portion, avoiding the magnified reading area which can distort depth perception. Most individuals adapt to these visual changes within one to two weeks by consistently wearing the new glasses.