Spectacle Lens Prescriptions Explained
For many people there is a lot of confusion surrounding their spectacle prescription. In this article we will explain what an optometrist writes on your prescription and how this affects your vision.
On your prescription you will have the following boxes in which something may be written:
- Sphere (could be labelled sph)
- Cylinder (could be labelled cyl)
- Addition (could be labelled Add, Reading Add or any combination)
This could be thought of as your base prescription. The number in this box will always be prefixed by a plus (+) or minus (-) sign. If there is a plus you are longsighted, if there is a minus you are short sighted. Optometrists never write zero (or a nought) as the number could be misread. Instead, if your Sphere prescription is zero they will either write ‘plano’ or ‘∞’. The higher the number the more long or short sighted you are. Typically values range between plano (zero) and 3 but values much higher than this can sometimes be found.
This part of the prescription is responsible for correcting any astigmatism you may have. Rather than blurred vision per se, people with astigmatism will sometimes notice that their vision is distorted. Most people have some degree of astigmatism and is simply another element to a prescription, nothing to worry about. The value in this box could be thought of as extra power in the lens, but one which only works in one direction.
Your prescription may not have anything written in this box. If it does it will follow similar rules as the values in the sphere box i.e. will have a plus or minus sign in front of the number. Values generally range from 0.25 to 2 but again values can go significantly higher than this.
There will only be something written in this box if there is something in the cylinder box. This part of the prescription is not a power, but simply tells us in which direction the cylinder acts. Values range between 1 and 180 degrees.
Once again there may not be anything written in this box. This part of the prescription corrects any muscle imbalances you may have between your two eyes. There will usually be 2 elements to this part of the prescription- a number, usually between 0.25 and 6 and a second element which may be a word (up, down, in, out) or an arrow pointing up, down, left or right. If you think of a prism as a triangular piece of plastic the number denotes how fat the base of the prism is and the word or arrow tells us how the base of the prism is to be positioned within the lens.
Once again you may not have anything written here. Any value will be prefixed by a plus sign and tells us how much extra sphere power is needed to enable you to focus for reading. Up to the age of 40 years old the chances are your eyes are able to do the extra focussing without any help. As you pass 40 years old there is an increasing chance you will need extra help built into your glasses to help you focus on near objects.
When confusion sets in…
Unfortunately for the layperson there are 2 different ways a prescription can be written, which can lead to confusion if you are trying to look at your own prescription to ascertain whether there has been a change or not. There is no universal convention for which one ‘should’ be used.
There is a formula to convert one way to the other which would not be particularly helpful in this forum. You can, however, tell fairly easily if you are looking at prescriptions written in the same form. The key is to look at the figure written in the ‘cylinder’ box. If the sign is the same in the two prescriptions you are comparing e.g. both have minus signs next to the number, the prescriptions have probably been written in the same form. If the figure has a plus in the cylinder box on one prescription but a minus in the other the chances are you won’t easily be able to compare and the numbers in the rest of the prescription.
The two prescription could look drastically different but actually the same. The best thing to do would be to bring your prescription in to your local Yates & Suddell and we will be happy to advise you further..