An eye prescription is a detailed record of the lens powers required to correct an individual’s vision. Among the various values listed, the “ADD” value represents a specific correction. This article will explain what “ADD” signifies, why it becomes necessary with age, and how it is incorporated into eyeglasses to provide clear vision across different distances.
Decoding the “ADD” Value
The “ADD” on an eye prescription stands for “Addition,” referring to the additional magnifying power needed for clear near vision. This value is always a positive number, measured in diopters, and typically ranges from +0.75 to +3.50. It represents the extra strength applied to the lower portion of multifocal lenses to help the eyes focus on close-up tasks like reading a book or viewing a smartphone.
This “ADD” power differs from other values on a prescription, such as “SPH” (Sphere) or “CYL” (Cylinder). The “SPH” value corrects for nearsightedness or farsightedness, addressing general distance vision. The “CYL” value, along with “AXIS,” corrects for astigmatism. In contrast to these, “ADD” specifically provides magnification for close-range activities.
Why “ADD” Becomes Necessary
The physiological reason behind the need for an “ADD” value is a natural age-related condition known as presbyopia. Presbyopia typically begins around the age of 40 and progresses gradually. As individuals age, the crystalline lens inside the eye, which is normally soft and flexible, gradually loses its elasticity and ability to change shape. This hardening of the lens reduces its capacity to accommodate or focus on nearby objects, leading to blurry close-up vision.
Presbyopia is a universal process and a normal part of aging, not a disease. It is distinct from general farsightedness (hyperopia), which is a refractive error present from birth where distance vision can also be affected.
How “ADD” Functions in Eyeglasses
The “ADD” power is integrated into various types of multifocal eyeglasses to provide clear vision at multiple distances. These include bifocals, trifocals, and progressive lenses. In bifocals, the lens is divided into two distinct sections: the upper portion for distance vision and a visible segment in the lower part that contains the “ADD” power for near vision.
Trifocals expand on this by adding a third segment for intermediate vision, located between the distance and near portions. Progressive lenses, also known as varifocals, offer a seamless transition of power from distance at the top, through an intermediate zone in the middle, to near vision at the bottom, without any visible lines. The “ADD” power is gradually incorporated into the lower part of these lenses, ensuring that individuals can comfortably shift their focus between far, intermediate, and near objects using a single pair of glasses.