How Do I read my glasses prescription?
Deciphering your prescription
Glasses prescriptions can often seem like a concoction of numbers and letters, which leaves most of us scratching our heads. However, once you know what you're looking at then it all becomes quite straightforward and it all falls into place. Let’s break down the anatomy of a glasses prescription, clarify whether you can use your glasses prescription for contact lenses and go through all the types of spectacle lenses that exist.
What do the symbols on my prescription mean?
There are a few main terms that you will see on every glasses prescription:
1. OD and OS: OD stands for the Latin "oculus dextrus" and refers to your right eye. OS means "oculus sinister" and refers to your left eye.
2. SPH (Sphere): This shows the amount of lens power, measured in diopters (D), prescribed to correct your nearsightedness or farsightedness. If the number appearing under this category has a minus sign (-), you are nearsighted; if it has a plus sign (+), you are farsighted.
3. CYL (Cylinder): This shows the amount of lens power for astigmatism. If nothing appears in this column, either you have no astigmatism, or it's so insignificant at the moment that it doesn't need to be corrected.
4. AXIS: If there's a cylinder power, there must be an axis, which points out the orientation of astigmatism. This number, which is between 1 and 180, shows the orientation of the cylindrical power in degrees.
Other uncommon symbols
Certain symbols may also appear on your glasses prescription such as:
"BAL" which stands for balance, means that there is no vision correction in that eye, it's normally used when one eye needs correction and the other doesn’t.
"DS" stands for "diopters sphere," which means your vision correction is the same in all meridians of your eye.
"PL” stands for Plano which means there is no nearsighted or farsighted correction needed.
Prism: If you have issues such as strabismus (crossed eyes) or diplopia (double vision), your eye doctor might include a prism in your prescription. The prism value in your prescription is measured in prism diopters (p.d. or Δ). The base (Base) means the thickest edge of the prism, which could be Base Up (BU), Base Down (BD), Base In (BI), or Base Out (BO).
Sometimes, for minor alignment issues, an optometrist might include the prism power in the regular prescription without writing it out separately. This practice doesn’t happen very regularly and could lead to confusion if another optometrist interprets the prescription. In cases where the prism correction is very slight, it is the easiest way to provide the necessary correction without complicating the lens making process.
Add: 'Add' is short for Addition and is the extra magnification needed for reading and close-up work. This number, measured in diopters, represents the additional corrective power that transforms the distance prescription in your glasses into a prescription for near tasks.
The 'Add' figure is usually the same for both eyes and can range from +0.75 to +3.00 D. This value is often not added to the Sphere power but written separately to inform the lens manufacturer that multifocal lenses are needed but in some cases, the 'Add' value could be factored into the Sphere measurement for reading glasses.
While both 'Prism' and 'Add' values can technically be added to other parts of the prescription, most optometrists choose to write them out separately so that there are no mistakes when it comes to making your glasses.
Most common prescription layouts from opticians
There are typically two main layouts used by opticians. The first starts with the Sphere, followed by Cylinder, and finally Axis. The second starts with Cylinder, followed by Axis, and then Sphere. Most optometrists in the UK use the first format.
Putting It All Together
Reading an eye prescription is like piecing together a puzzle. For instance, if your prescription reads -2.00 -1.50 x 180 in your right eye and -1.75 -0.75 x 135 in your left eye, here's what it means:
- Your right eye has a near-sightedness (sphere) correction of -2.00 dioptres.
- Additionally, there's an astigmatism correction (cylinder) of -1.50 dioptres in your right eye.
- The axis of 180 degrees indicates how the astigmatism correction should be aligned.
- In your left eye, there's a near-sightedness correction of -1.75 dioptres.
- Your left eye also has an astigmatism correction of -0.75 dioptres, with an axis of 135 degrees.
Can I use my glasses prescription for contact lenses?
Unfortunately not. While your glasses and contact lens prescriptions aren't interchangeable, they are closely related. They won't match because glasses are positioned a very short distance from your eyes, whilst contact lenses sit directly on the surface of your eyes. That changes the type, and amount of correction required. So, although a glasses prescription is closely related to a contact lens prescription you'll still need a separate contact lens fitting to get the right prescription.
What types of spectacle lenses are there?
Spectacle lenses come in several types to help with a specific vision requirement:
1. Single Vision Lenses: These lenses have the same focal power throughout and can correct nearsightedness, farsightedness, or astigmatism.
2. Bifocal Lenses: The upper part of the lens is usually for distance vision, while the lower part is for near vision.
3. Trifocal Lenses: As the name suggests, these lenses have three points of focus – for distance, intermediate, and near vision.
4. Progressive Lenses: Similar to bifocal or trifocal lenses but with a smoother transition between different focal points.
5. Photochromic Lenses: These lenses darken when exposed to UV radiation, turning them into sunglasses pretty much when you’re outdoors.
6. Aspheric Lenses: These lenses have a more complex front surface, they are thinner and lighter and give a higher vision correction than most of the other lenses.
By understanding your glasses prescription, which is the first step to making sure you receive the right eyewear for your needs, you can discuss your prescription with your optician or eye care professional. And remember, regular eye examinations are vital for keeping your vision healthy and catching any potential visual or health issues early.
Author: John Dreyer Optometrist Bsc(Hons), MCOPTOM, DipCLP
Created: 7 Sep 2023, Last modified: 18 Nov 2023