An eye prescription is a technical document detailing the lens power required to correct vision errors, including distance vision and astigmatism. For many people, a unique value labeled “ADD” appears, signifying an extra lens strength needed for viewing objects up close. The presence of the ADD value indicates a need for multi-focal lenses, which are designed to provide clear vision at two or more different distances.
The Purpose of the ADD Value
The abbreviation “ADD” stands for “Addition” or “Added Power,” representing the supplementary magnifying power built into the lens for near tasks. This power compensates for presbyopia, a common age-related condition where the eye’s natural crystalline lens becomes less flexible over time. In a younger eye, the lens changes shape easily to shift focus from distant to near objects, a process called accommodation. As the lens hardens, this ability to accommodate for near vision is lost, making tasks like reading difficult.
The ADD power is always recorded as a positive value, measured in diopters, because it adds magnifying power to the existing distance prescription. This value is determined during an examination to ascertain the exact extra focus required for comfortable reading distance. For example, if the distance correction is -1.00 diopter and the ADD is +2.00 diopters, the final reading power is +1.00 diopter.
The ADD value is typically the same for both eyes since presbyopia affects both eyes equally. This single number provides the specific magnification needed to restore the ability to focus at a standard reading distance, generally about 16 inches away.
How ADD Power is Integrated into Lenses
The single ADD value is used to create multi-focal lenses that blend near and distance correction. This added power is placed within the lower portion of the lens, corresponding to the natural downward gaze used when reading. The method of integration varies depending on the lens type chosen.
Bifocal lenses are characterized by a clear, visible line that separates the distance correction in the upper portion from the near correction below. The segment containing the ADD power is distinct and provides an abrupt shift in focus from far to near. This design ensures the entire lower segment has the full added power needed for reading.
Trifocal lenses expand on this design by including a third segment, typically located just above the near-vision area. This middle segment incorporates an intermediate power, usually half of the full ADD power, to correct vision at arm’s length, such as for computer work. Like bifocals, trifocals have visible lines separating the three zones of correction.
Progressive Addition Lenses (PALs), often called “no-line bifocals,” integrate the ADD power using a corridor of gradually increasing power. The distance correction is positioned in the top of the lens, and the ADD power is gradually introduced as the wearer’s gaze moves down the lens. This gradual change provides a smooth transition between distance, intermediate, and near vision without any visible lines or abrupt power shifts.
The final optical power in the reading zone is calculated by combining the distance power with the ADD power. The ADD value is superimposed onto the distance prescription, ensuring the wearer achieves clear focus at all necessary ranges.
Progression and Stability of ADD Power
The need for an ADD power typically begins to manifest around the age of 40 to 45, which is when the hardening of the eye’s lens first becomes noticeable enough to interfere with near tasks. This initial added power is usually quite low, often starting in the range of +0.75 diopters. The initial symptoms are often a need to hold reading material farther away to achieve focus.
Because the underlying physical change is progressive, the required ADD power increases over time. For the next two decades, eye care professionals adjust the ADD value to accommodate the increasing loss of natural focusing ability. This gradual increase generally progresses in small increments, often in steps of +0.25 or +0.50 diopters.
The typical spectrum of the ADD power ranges from +0.75 D up to a maximum of approximately +3.00 D. This maximum value represents the point at which the eye’s natural accommodation is nearly exhausted. Most people find that the required ADD power stabilizes in their mid-sixties, meaning significant increases are no longer necessary.
Once the ADD power stabilizes, the correction for near tasks remains relatively constant, though other components of the prescription may still change slightly. Regular eye examinations are necessary to ensure the prescription remains accurate and to monitor overall eye health.