The ability of the eye to focus on nearby objects naturally diminishes over time, a condition known as presbyopia. This gradual loss of focusing flexibility, which often begins around the age of 40, makes reading and other close-up tasks increasingly difficult. Corrective lenses for near vision are required to compensate for this natural change. The specific optical correction needed to restore clear near vision is calculated by an eye care professional and is referred to as the “Add Power.”
Understanding the “Add Power”
The term “ADD” on a glasses prescription stands for “Addition” power, representing the extra magnification required for comfortable near work. This power is necessary because the crystalline lens inside the eye stiffens with age, preventing the lens from changing shape sufficiently to focus light from close distances onto the retina. The Add power is always recorded as a positive, or plus, power, which effectively shifts the focal point forward to allow clear vision at a typical reading distance.
This additional power is measured in diopters and is typically the same for both the right eye (OD) and the left eye (OS). The values usually fall within a range of approximately +0.75 D to +3.50 D, increasing incrementally as the eye’s natural focusing ability continues to decline.
Professional Determination of Near Vision Needs
The calculation of the necessary Add power is a precise clinical process focused on determining the gap between the eye’s remaining focusing ability and the visual demand of the patient’s intended working distance. The primary method involves measuring the eye’s current Amplitude of Accommodation (AoA), which is the maximum amount of power the eye can still exert to focus on a near object. This is often measured using a “push-up” method, where a target is moved closer to the eye until it blurs, with the distance then converted to diopters.
Eye care professionals use the AoA measurement to calculate the required lens power, ensuring the patient only uses a portion of their remaining focusing capacity to prevent eye strain. A common clinical guideline is to prescribe an Add power that leaves at least one-half of the patient’s measured AoA in reserve. For instance, if a patient has an AoA of 3.00 D, the reading prescription only requires 1.50 D of the patient’s own effort, leaving the other 1.50 D in reserve for sustained comfort.
The required power is directly related to the patient’s customary working distance, which is converted into a dioptric value. For a typical reading distance of 40 centimeters, the dioptric demand is 2.50 D (calculated as the reciprocal of distance in meters: 1/0.40 m). The tentative Add power is determined by subtracting the patient’s reserved AoA from the dioptric demand of the working distance. For example, if the 40 cm distance requires 2.50 D and the patient is reserving 1.50 D of their own AoA, the resulting Add power would be +1.00 D.
While age-based charts serve as useful starting references, the final Add power is always refined through subjective testing. The eye care professional tests the patient with trial lenses, using the weakest plus power that allows the patient to comfortably read the smallest print at their preferred near distance. This customized approach ensures the final prescription provides optimal visual comfort and clarity for the individual’s specific needs and activities.
Interpreting the Final Prescription
The calculated Add power is integrated into the full spectacle prescription, which follows a standardized format detailing the correction for distance vision. The prescription grid typically includes columns for Sphere (SPH), Cylinder (CYL), and Axis, describing the distance correction for nearsightedness, farsightedness, and astigmatism. The Add power is usually listed in its own box or row, labeled “ADD,” and is the value added to the distance prescription for near viewing.
To find the total power of the reading portion of the lens, the Add power is algebraically summed with the distance Sphere power. For example, a patient with a distance prescription of -1.00 D (SPH) and an ADD of +2.00 D will have a total near vision power of +1.00 D. This calculation is performed by the optical lab when creating the lenses.
The application of the Add power within the lens structure varies depending on the lens type chosen. In traditional bifocal lenses, the Add power is contained within a visible segment at the bottom of the lens. For modern progressive lenses, which lack visible lines, the Add power gradually increases along a vertical corridor, providing a smooth transition from the distance power at the top to the full near power at the bottom.