Sometimes the prescription that you receive from your optician after an eyetest is difficult to read (seems like their handwriting can be as bad as the doctors !), and it can be hard to interpret correctly to transcribe into an online glasses site to buy your new glasses online.

It is not (necessarily) because they are trying to make it difficult for you to buy glasses online, it is usually because there are several different ways for the glasses prescription to be written.

When you first look at your spectacles prescription, there will be a series of numbers and/or dashes under the box headings of OS and OD. These are Latin abbreviations for the left eye (OS) and the right eye (OD) – further confusing I know because these abbreviation stand for something that you are unlikely to hear in normal conversation - Oculus sinister and Oculus dextrus. There is an extra notation that is infrequently seen which is an indicator for something applying to both eyes – which is “OU”.

The easiest way to think of the values on your glasses prescription is to look at it and the further from zero that the value is (regardless of sign) is proportional to how your eyes perform relative to perfect vision. If the value is preceded  by a plus sign, so that you have a positive deviation – then you are long-sighted (or sometimes referred to as long-sighted). If the value is preceded  by a minus or negatve sign (sometimes written as a bar over the top of the number or value) then you are short-sighted (or near sighted).,

The values themselves are a measure in “diopters” which is a unit of measure used by opticians to indicate quantative correction necessary for the lens to correct your vision for your glasses. The vale may often have a “D” next to it to indicate that this is the value system being used for the spectacles or lens prescription.

In people, the total optical power of the eye at rest is approximately 60 diopters. This is made up by the cornea accounting for about 40 diopters and the lens of the eye itself for the balance. When the eye attempts to focus on an object, the lens in the eye is torsioned b ythe cilliary muscle which changes the shape of the lens to increase the optical power or the ability of the lens to focus. This process is governed by the cilliary muscle through ligamnets attached to the lens.

Like most things, as you get older, these muscles lose some of their strength or ability to changethe shape of the lens – and so the eye loses some of its ability to focus – particularly at the extremes.

Children (or young adults) have the ability to to change the power by about 20 diopters, which then decreases as we get older to low single digits at age 50 or so.

Convex lenses are used by opticians in glasses to correct far-sight or to allow people to focus more easily on objects that are close (usually for reading). These lenses have a positive value in diopters.

Concave glasses or lenses have a negative value and correct for short-sight or improve the ability of the eye to focus at long(er) ranges.

A moderate range for these values is up to approximately 3 diopters in each mode. Lenses are usually available from stock at opticians or glazing labs in 0.25 incremenets to accommodate the majority of people for new glasses.

have positive dioptric value and are generally used to correct hyperopia (farsightedness) or to allow people with presbyopia (the limited accommodation of advancing age) to read at close range. Concave lenses have negative dioptric value and generally correct myopia (nearsightedness). Typical glasses for mild myopia will have a power of ?1.00 to ?3.00 dioptres, while over the counter reading glasses will be rated at +1.00 to +3.00 dioptres. Optometrists usually measure refractive error using lenses graded in steps of 0.25 dioptres.

Sometimes there are three numbers in the glasses prescription – the third being used to define astigmatism. These numbers then are usually written as  S x C x Axis.

Astigmatism is essentially a measure of refractive error and is quite common – resulting in a lack of ability to focus on fine details of objects – but is easily corrected with the proper prescription to amend the degree of radial curvature across the lens.

The S refers to the “spherical” portion of the prescription, which is the degree of short or long sight – or the amount of correction required by the lens.

The C refers to the “cylinder” or astigmatism, and can be a negative or a positive number. It is also measured in diopters and refers to the amount of astigmatism you may have.

The Axis is a number anywhere between 0 and 180 degrees. It defines and measures the vector of astigmatism – or the orientation of it across the lens. This is a complicating factor in manufacture of the lens, as it is necessary but not sufficient know how much correction is required – you also need to know how that correction should be placed across the curvature of the lens.

Here are two examples of what prescriptions for glasses or lenses to correct sight with astigmatism could look like:

-1.50 +1.25 x 180

+3.00 +2.00 x 45

The first prescription means that the person has 1.5 diopters of nearsightedness with 1.25 diopters of astigmatism. and an axis of 180 degrees.

The second prescription means that the person has 3.0 diopters of farsightedness, 2 diopters of astigmatism and an axis of 45 degrees.

 

Tags: astigmatism, Buying Glasses Online, diopters

This entry was posted on Wednesday, October 14th, 2009 at 2:24 pm and is filed under Glasses, Lenses. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.