When reviewing an eye prescription, the rows of numbers and abbreviations can be confusing. One frequently misunderstood term is the abbreviation “NV.” This term is a specific instruction to the lens laboratory, relating directly to your ability to see clearly up close. Understanding what NV means and how its associated value is determined is key to ensuring you receive the correct corrective lenses.
Defining NV and Near Vision Requirements
The abbreviation NV stands for Near Vision, indicating the lens power needed for close-up tasks. This correction is required for activities performed at a reading distance, typically 14 to 16 inches (about 40 centimeters) from the eye. NV correction is necessary due to presbyopia, a common, age-related condition.
Presbyopia occurs when the eye’s natural lens stiffens and loses flexibility, making it difficult to focus on nearby objects. This condition usually begins around age 40. The NV value measures the correction needed to compensate for this lost focusing ability, restoring clear sight for reading or detailed work.
The Role of ADD Power in Prescriptions
The numerical value associated with the NV requirement is found in the column labeled “ADD,” which stands for “Addition” or “Added power.” This number represents the extra magnifying power that must be added to the distance prescription to achieve clear focus at near range. The ADD power is measured in diopters (D), the standard unit for lens strength.
This value is always positive, regardless of whether the distance prescription is positive (farsighted) or negative (nearsighted). The typical range for ADD power is between +0.75 D and +3.50 D, and it is usually the same for both eyes. The optometrist determines this specific power during an eye examination by having the patient read a near-vision chart at their comfortable working distance.
As presbyopia progresses over time, the ADD power value will likely increase in small increments during subsequent eye exams. The final ADD number ensures that the total power of the lens provides the exact magnification required for comfortable near tasks.
How NV Values are Applied to Corrective Lenses
The NV or ADD value guides the fabrication of a patient’s multifocal or reading lenses. The simplest application is in single-vision reading glasses, which contain only the ADD power across the entire lens surface. For individuals who also require distance correction, the NV value is incorporated into a multifocal lens design.
One common option is a bifocal lens, where the ADD power is placed in a distinct, visible segment in the lower portion of the lens. Trifocal lenses add a third section with a slightly lower power for intermediate tasks, such as viewing a computer screen. Both of these lens types have noticeable lines separating the different viewing zones.
Alternatively, the NV value is used to create progressive lenses, also known as varifocals. In this design, the ADD power is seamlessly blended into a vertical corridor, transitioning smoothly from the distance correction at the top to the full near correction at the bottom. This blending provides a natural, line-free appearance and allows for clear vision at all distances.