Eyeglass prescription

An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.

If an examination indicates that corrective lenses are appropriate, the prescriber generally provides the patient with an eyewear prescription at the conclusion of the exam. The parameters specified on spectacle prescriptions vary, but typically include the power to which each lens should be made in order to correct blurred vision due to refractive errors, including myopia, hyperopia, astigmatism, and presbyopia. It is typically determined using a phoropter asking the patient which lens is best, computer automated refractor, and through the technique of retinoscopy.

Similar to medical prescriptions, eyeglass prescriptions are written on paper pads that frequently contain a number of different abbreviations and terms:

• DV is an abbreviation for distance vision. This specifies the part of the prescription designed primarily to improve far vision. In a bifocal lens, this generally indicates what is to be placed in the top segment.
• NV is an abbreviation for near vision. This may represent a single-vision lens prescription to improve near work, or the reading portion of a bifocal lens. Some prescription forms use ADD in place of NV with a single box to indicate the additional refractive power to be added to the spherical of each eye.
• OD is an abbreviation for oculus dexter, Latin for right eye. Oculus means eye. In some countries, such as the United Kingdom RE (right eye), LE (left eye), and BE (both eyes) are used. Sometimes, just right and left are used.
• OS is an abbreviation for oculus sinister, Latin for left eye.
• OU is an abbreviation for oculi uterque, Latin for both eyes.
• A spherical correction corrects refractive error of the eye with a single convergent or divergent refractive power in all meridians.
• A cylindrical correction corrects astigmatic refractive error of the eye by adding or subtracting power cylindrically in a meridian specified by the prescribed axis.
• The axis indicates the angle in degrees of one of two major meridians the prescribed cylindrical power is in.

Most eyeglass prescriptions will contain values here. The spherical and cylindrical columns contain lens powers in diopters (see below).

• Prism and Base are usually left empty, as they are not seen in most prescriptions. Prism refers to a displacement of the image through the lens, and is used to treat eye muscle imbalances or other conditions (see vergence dysfunction) that cause errors in eye orientation or fixation. Prism correction is measured in "prism diopters", and Base refers to the direction of displacement.
• Pupillary Distance (PD) is the distance between pupils, usually given in millimeters.
• Back vertex distance (BVD ) is the distance between the back of the spectacle lens and the front of the cornea (the front surface of the eye). This is essential in higher prescriptions (usually above ±4.00D) as slight changes in the distance between the spectacles and the eyes above this level can cause the patient to perceive a different power, leading to blur and/or other symptoms.

Background

Blur is the subjective experience or perception of a defocus aberration within the eye. Blur may appear differently depending on the amount and type of refractive error. The following are some examples of blurred images that may result from refractive errors:

Blur is corrected by focusing light on the retina. This may be done with eyeglasses or contact lenses, or by altering the shape of various eye structures via refractive surgery or special contact lenses.

The eye chart is the background used by eye doctors to compare the patient's visual acuity with the one of other human beings. Although there are many variations of the eye chart, the standard one is the Snellen eye chart, which was developed by Dutch eye doctor Hermann Snellen in the 1860s. .

Individuals who are not able to read letters for various reasons including being too young to know the alphabet or having a handicap, eye doctors may use what is called the tumbling E chart. This type of chart is a variation of the Snellen chart and it shows the capital letter E, at different sizes and rotated in increments of 90 degrees. The scale of the tumbling E chart is the same as with the standard Snellen chart. The eye doctor, in this case, will ask the person being tested to use either hand (with their fingers extended) to show which direction the "fingers" of the E are pointing: right, left, up or down.^[3]

In the United States, a 20/20 visual acuity is considered normal. This means that the chart is normally placed at 20 feet distance from the person who is being tested. 20/20 visual acuity is considered normal vision for individuals but not perfect as some individuals, although rarely, can see at 20 feet what others can see at 10. The poorest visual acuity is 20/200 and a person with the best-corrected vision, or vision once wearing corrected lenses, of 20/200 is normally considered legally blind. Individuals with 20/200 vision are normally able to read only the first letter on the chart. Usually the 20/20 line of letters is fourth from the bottom, with 20/15, 20/10 and 20/5 below that. Not many people have 20/10 or better visual acuity, but many animals do, especially birds of prey, which have been estimated to have an acuity of 20/5 or even better. In the United States individuals who want to get their driver license without a corrective lenses restriction must have at least 20/40 visual acuity.

Lens power

The values indicated in the sphere and cylinder columns of an eyeglass prescription specify the optical power of the lenses in diopters, abbreviated D. The higher the number of diopters, the more the lens refracts or bends light. A diopter is the reciprocal of the focal length in meters. If a lens has a focal length of ¹⁄₃ meters, it is a 3 diopter lens.

A +10 diopter lens, which has a focal length of 10 centimeters, would make a good magnifying glass. Eyeglass lenses are usually much weaker, because eyeglasses do not work by magnifying; they work by correcting focus. A typical human eye without refractive error has a refractive power of approximately 60 diopters.

Stacking lenses combines their power by simple addition of diopter strength. A +1 diopter lens combined with a +2 diopter lens forms a +3 diopter system.

Lenses come in positive (plus) and negative (minus) powers. Given that a positive power lens will magnify an object and a negative power lens will minify it, it is often possible to tell whether a lens is positive or negative by looking through it.

Positive lenses cause light rays to converge and negative lenses cause light rays to diverge. A negative lens combined with a positive lens results in a system with a power equal to the sum of the two lenses, so a −2 lens combined with a +5 lens forms a +3 diopter system.

A −3 lens stacked on top of a +3 lens looks almost like flat glass, because the combined power is 0.

In science textbooks, positive lenses are usually diagrammed as convex on both sides; negative lenses are usually diagrammed as concave on both sides. In a real optical system, the best optical quality is usually achieved where most rays of light are roughly normal (i.e., at a right angle) to the lens surface. In the case of an eyeglass lens, this means that the lens should be roughly shaped like a cup with the hollow side toward the eye, so most eyeglass lenses are menisci in shape.

The most important characteristic of a lens is its principal focal length, or its inverse which is called the lens strength or lens power. The distance from the lens to that point is the principal focal length of the lens. For a double concave lens where the rays are diverged, the principal focal length is the distance at which the back-projected rays would come together and it is given a negative sign

Amount of refractive error and degree of blur

Approximation of blur seen by a patient.

The leftmost image above shows a Snellen eye chart as it might be seen by a person who needs no correction, or by a person who is wearing eyeglasses or contacts that properly correct any refractive errors he or she has.

The images labelled 1D, 2D, and 3D give a very rough impression of the degree of blur that might be seen by someone who has one, two, or three diopters of refractive error. For example, a nearsighted person who needs a −2.0 diopter corrective lens will see something like the 2D image when viewing a standard eye chart at the standard 20-foot distance without glasses.

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.

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.

Buying Glasses
The actual purchase process for glasses, for most people, involves visiting an eyeglass store, getting a “free” test from an optometrist, and then buying their expensive glasses.

Maintaining Glasses
Maintaining glasses isn’t too hard, either. Just remember that you don’t need to replace the frames and lenses every and that with a bit of effort, frames can last for many years and lenses can last for several years, too, if you don’t face a prescription change. Here are some tips for maintenance.

Wash them regularly I wash mine every other day or so in warm, soapy water, then dry them with a soft cloth. This keeps the lenses quite clean and thus reduces the chances for scratching. Don’t use anything scratchy to dry the lenses – avoid paper towel drying like the plague.

If they get scratched… Try polishing the scratch with some eyeglass polish. You can usually find a can of this for $8 to $10 at the eyeglass store – ask for help before buying because some brands don’t work well on glasses with various coatings on the lens. This is far, far cheaper than replacing your lens, because a can of polish should be able to last for years.

You can try baby oil as well – this often removes small scratches on lenses. Just buff it off with a very soft cloth. I have heard of other solutions, but they all work only on specific lens types.

If the frame gets broken… Superglue is often your best friend for longer pieces that are broken in the middle.

If a screw gets loose… Screw it back into place, then apply a drop of nail polish to the top of the screw. That’ll hold it right where it should be.

Features

Bifocals or trifocals are more costly than standard lenses, and no-line bifocals are priced even higher. Having extras such as scratch-resistant coating or anti-glare coating will edge up the price as well.

price of eyeglasses is still highly connected with other features, including varied coatings. Shatter-resistant coating, anti-reflective coating and others frequently charge customers significantly additional charges. In a few cases, the price of a special pair of glasses equipped with some extra features can be several times higher than a normal pair. There's a much bigger opening among differing types of fashion frames.

Along with the types of lenses and frames chosen, choosing where to purchase eyeglasses is a big factor in cost. Generally, warehouse clubs and internet retailers have lower prices than optometrists' offices or chain stores. However, for quality follow-up service you might consider paying a little more if you can afford to.