Distant Vision and Near Vision
The DV portion of the prescription describes the corrections for distant vision. For most people under forty years of age, this is the only part of the prescription that is filled in. The NV or near-vision portion of the prescription is blank because a separate correction for near vision is not needed.
The NV portion is used in prescriptions for bifocals.
In younger people, the lens of the eye is still flexible enough to accommodate over a wide range of distances. With age, the lens hardens and becomes less and less able to accommodate.
This is called "presbyopia"; the presby- root means "old" or "elder". (It is the same root as in the words priest and presbyterian.)
The hardening of the lens is a continuous process, not something that suddenly happens in middle age. It is occurring all along. All that happens around middle age is that the process progresses to the point where it starts to interfere with reading. Therefore almost everybody needs glasses for reading from the age of 40–45.
Because young children have a wider range of accommodation than adults, they sometimes examine objects by holding them much closer to the eye than an adult would.
This chart (which is approximate) shows that a schoolchild has over ten diopters of accommodation, while a fifty-year-old has only two. This means that a schoolchild is able to focus on an object about 10 cm (3.9 in) from the eye, a task for which an adult needs a magnifying glass with a magnification of about 3.5.
The NV correction due to presbyopia can be predicted using the parameter age only. The accuracy of such a prediction is sufficient in many practical cases, especially when the total correction is less than 3 diopters. See also the following calculator for computing this correction.
When someone accommodates, they also converge their eyes. There is a measurable ratio between how much this effect takes place (AC:A ratio, CA:C ratio). Abnormalities with this can lead to many orthoptic problems.
Read more about this topic: Eyeglass Prescription
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