When reviewing an eyeglass prescription, the columns of numbers and abbreviations can seem confusing, especially for those unfamiliar with optometric terminology. Most people recognize the figures for distance vision, but the abbreviation “NV” often causes uncertainty. This detail represents a significant change in vision that affects nearly everyone as they age. Understanding what NV means helps clarify the purpose of any new multi-focal lenses.
Decoding the Near Vision (NV) Measurement
The abbreviation NV stands for Near Vision or Near Point. It specifies the lens power required for seeing clearly at close range, such as reading a book or looking at a smartphone. This measurement is not part of the core distance vision correction but represents an additional magnifying power, listed under the column labeled “ADD,” short for Addition.
The ADD power is always a positive number, measured in diopters, indicating increased lens magnification. For most adults, the range falls between +1.00 and +3.00 diopters. A higher number signifies a greater need for magnification to achieve clear close-up focus.
The NV or ADD power is commonly the same for both the right eye (OD) and the left eye (OS). Since the underlying focusing issue is symmetrical, the additional power required is typically identical. This uniform value is then added to the distance prescription to calculate the full power needed for near viewing tasks.
The Role of Presbyopia in NV Prescriptions
The physiological reason NV power becomes necessary is presbyopia, a natural part of the aging process. As a person reaches their early to mid-forties, the lens inside the eye loses flexibility. This loss impairs the eye’s ability to quickly change focus between distant and near objects, a mechanism called accommodation.
The lens stiffens with age, making it difficult for the ciliary muscles to focus light rays onto the retina at close distances. This results in the need to hold reading material further away to see the print clearly. Presbyopia affects everyone, regardless of their previous distance vision.
The NV or ADD power is an optical correction intended to compensate for this lost focusing ability. The measurement is determined by testing the patient’s vision at their comfortable reading distance, typically around 40 centimeters. The resulting diopter value is the additional power required to bring the close-up image back into sharp focus.
How NV Power is Incorporated into Lenses
The NV power, recorded as the ADD value, is the instruction used by lens manufacturers to create multi-focal eyewear. This number is the basis for producing single-vision reading glasses, bifocals, or progressive lenses. For dedicated reading glasses, the ADD power is combined with the distance correction to create a lens suited entirely for near work.
In a bifocal lens, the NV power is incorporated into a distinct segment in the lower portion, separated from the distance vision area by a visible line. The ADD value represents the difference in power between the upper and lower segments. This design offers two separated fields of vision: one for far and one for near.
The most common application is in progressive lenses, which are multi-focal lenses without a visible line. The ADD power creates a gradual corridor of power increase from the distance portion at the top, through an intermediate zone, to the full near power at the bottom. This intermediate zone, used for tasks like viewing a computer screen, is often calculated using about half of the total NV power.