The term “Addition” or “ADD” on a prescription for glasses represents a specific correctional power required for clear near vision. This value is mathematically added to the main distance prescription to create a single lens capable of correcting vision at multiple distances. It indicates that the patient needs assistance to comfortably focus on objects up close, such as reading material or a smartphone screen. The addition power is a significant measurement for anyone receiving a multifocal lens.
Defining the Addition Power
The Addition power is a positive value, measured in diopters, representing the extra magnifying strength incorporated into a lens for near-vision tasks. It is uniformly applied to both the right and left eyes in almost all cases. This power allows the eye to focus clearly at a typical reading distance, generally 14 to 16 inches from the face.
The ADD power acts as a built-in magnifying glass layered over the existing distance correction. For example, if the distance prescription is -2.00 diopters and the ADD power is +2.00 diopters, the total power in the near-vision segment becomes 0.00 diopters. This extra power compensates for the eye’s inability to naturally increase its focusing power for close objects. The measurement typically ranges between +0.50 and +3.50 diopters, increasing as natural focusing ability decreases with age. The ADD value is a fixed amount that is used only in the lower or near-vision portion of a multifocal lens design.
The Vision Condition Requiring the Addition
The physiological condition necessitating the Addition power is presbyopia, a normal, age-related decline in the eye’s focusing ability. This condition is an inevitable consequence of the aging process, not a disease. Presbyopia typically begins to manifest around age 40, becoming progressively more noticeable until it stabilizes around the mid-60s.
Presbyopia involves the crystalline lens inside the eye, which loses flexibility and becomes more rigid over time. In a younger eye, the ciliary muscle contracts to change the shape of the soft lens, allowing the eye to adjust focus from far to near objects, a process called accommodation. As the lens hardens, it resists this shape change, limiting the eye’s ability to produce the necessary power for clear near vision. This results in the common symptom of needing to hold reading material at arm’s length. Presbyopia is distinct from other refractive errors like myopia (nearsightedness) or hyperopia (farsightedness).
Lens Types That Incorporate the Addition
The Addition power is incorporated into lenses through various multifocal designs to provide vision correction at multiple distances. Bifocal lenses are the most traditional application, featuring two distinct zones of power separated by a visible horizontal line. The upper portion holds the distance prescription, while the lower segment contains the full ADD power for reading.
Trifocal lenses expand upon this concept by adding a third segment above the near-vision area, providing correction for intermediate distances, such as computer work. Like bifocals, trifocals have visible lines separating the three power zones. Progressive Addition Lenses (PALs) offer the most seamless solution, incorporating the ADD power through a gradual, corridor-like change in lens power. This design eliminates the visible lines of bifocals and trifocals, transitioning smoothly from the distance prescription at the top to the intermediate power in the middle, and finally to the full near power at the bottom.