One of the most common terms found on an eye exam result is NV-ADD. This terminology signals a need for extra lens power to correct vision for tasks performed at close range. This value is a crucial component in prescriptions for anyone requiring vision correction at multiple distances.
Defining Near Vision Addition Power
The term NV-ADD stands for Near Vision Addition, often shortened to “ADD” on a prescription. It refers to the supplementary magnifying power needed in the lens to allow the eye to focus comfortably on objects held at a standard reading distance, typically 14 to 16 inches from the face.
This value is always a positive number, measured in diopters. It is added to the existing distance prescription (if one is present) to determine the full power required for near work. For example, a distance prescription of -2.00 diopters with an ADD of +1.50 diopters results in a total near power of -0.50 diopters. The ADD power is nearly always the same for both the right and left eyes. The typical range for this value starts around +0.75 diopters and can progress up to about +3.50 diopters.
The Underlying Need: Understanding Presbyopia
The requirement for an ADD power is directly linked to presbyopia, a natural, age-related process. This condition involves the gradual loss of the eye’s ability to focus on objects up close. It typically becomes noticeable around the age of 40 to 45 and continues to progress until about 60 to 65 years old.
Presbyopia occurs because the crystalline lens inside the eye slowly becomes less flexible and harder over time. In a younger eye, this lens changes shape—a process called accommodation—to bring near objects into sharp focus. As the lens loses its elasticity, the eye’s accommodative ability decreases, impairing the natural focusing mechanism.
How ADD Power Shapes Your Lenses
The NV-ADD measurement determines how the reading correction is physically incorporated into a multifocal lens design. This power is necessary for either bifocal or progressive lenses, which combine distance and near prescriptions into a single lens.
Bifocal Lenses
In a bifocal lens, the ADD power is confined to a distinct, visible segment in the lower portion of the lens. This design creates a sharp boundary between the distance vision area above and the near-vision segment below. The wearer must look down and through this lower segment to access the additional magnification for reading or close tasks.
Progressive Lenses
Progressive lenses, also known as varifocals, use the ADD power by creating a gradual change in magnification. The power increases smoothly through a vertical viewing channel, or “corridor,” running from the distance area at the top to the full ADD power at the bottom. This design offers a seamless transition that includes an intermediate vision zone, which is ideal for computer screens or dashboard viewing. A higher ADD number means a greater difference in power between the distance and near zones, which can result in a narrower clear viewing corridor and more peripheral blur.