Eyeglass Prescription

1. Distant vision and near vision
The DV portion of the prescription describes the corrections for distant vision. For most people under forty years old, 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.
For younger people, the lens of the eye is still flexible enough to accommodate over a wide range of distances. With age increased, the lens hardens and becomes more and more difficult to be accommodated.
This is called “presbyopia;” the “presby-” root means “old” or “elder.” (It is the same meaning 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 do.
It’s said 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. (4″) from the eye, a task for which an adult needs a magnifying glass with a rated power of about 3.5X.
The NV correction due to presbyopia can be predicted by 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.
2. Variations in prescription writing
There is a surprising amount of variations in the ways that prescriptions are written; the layout and terminology is not uniform.
When no correction is needed, the spherical power will sometimes be written as “0.00″ and sometimes as “Plano” or “Pl”. The lens, although not flat, is optically equivalent to a flat piece of glass, and has no refractive power.
When cylindrical correction is needed, the mathematics used to denote the combination of spherical and cylindrical power in a lens can be notated two different ways to indicate the same correction. One is called the plus-cylinder notation and the other is the minus-cylinder notation.
These two prescriptions are equivalent:

prescription glasses

Both of them specify a power of +2.00 diopters at the 90th (vertical) meridian and +3.00 diopters at the 180th (horizontal) meridian.
The first one specifies a +2.00 spherical component, which would give a power of +2.00 diopters in all meridians, and adds a +1.00 cylindrical component at 180 degrees (perpendicular to the axis indicated on the prescription, as explained under Axis). The result is +2.00 diopters at the 90th meridian and 2.00 + 1.00 = +3.00 diopters at the 180th meridian.
The second one specifies a +3.00 spherical component, which would give a power of +3.00 diopters in all meridians, and adds a -1.00 cylindrical component at 90 degrees. The result is 3.00 − 1.00 = +2.00 diopters at the 90th meridian and +3.00 diopters at the 180th meridian.
In practice, Optometrists tend to use minus-cylinder notation, whereas Ophthalmologists tend to use plus-cylinder notation.