Understand your glasses prescription!
We’ve all been there. You finish your eye exam and walk away with this paper that has a bunch of funny words on it and random numbers. What does it all mean?! Most eyeglass prescriptions are actually simpler to understand than they seem. Let’s take a look at an example straight from my clinic to get a better idea.
The best place to start is understanding what OD and OS mean. Simply put, OD is the right eye and OS is the left eye. OD is short for oculus dexter which means right eye in latin. OS is short for oculus sinister which means left eye in latin. In the medical domain, latin is the most often used language and many words utilized by Doctors are rooted in the latin language. On the top of the prescription you will notice more letters on there. SPH stands for sphere and CYL stands for cylinder. These are the main components of a prescription. Optical correction for the eyes is usually divided into 2 axes. One axis we call the spherical and the other cylinder. The spherical axis is corrected with a uniform lens that has the same power regardless of orientation. The cylinder axis is corrected with a lens that has power only in one axis. You may have noticed that after “CYL” the prescription says “AXIS”. The axis value helps those that make optical devices know how to orient the lens. Sphere values are either positive or negative. Positive sphere values indicate that the patient’s vision is farsighted, meaning they see well in distance. As you may have already guessed, negative sphere values indicate that a patient’s vision is nearsighted, meaning they see well up close. Cylinder values can also be positive or negative, but predominantly the negative is used in North America. Cylinder is also known as astigmatism, which impacts both near and distance vision, but distance more so.
The next abbreviation used on the top row is VA. This is short for visual acuity and refers to the level of vision present. You have likely heard of 20/20 and it refers to excellent eyesight. However, as vision gets worse the denominator of the fraction will increase (i.e. 20/100). So how is that value interpreted? Let’s use the example of 20/100. It means that the patient would have to be at 20 ft distance to see what an individual with no necessary optical correction and healthy eyes could see from 100 ft. Hence, you can see that as the denominator increases it would indicate worse vision overall. Most people can be corrected with optical devices to 20/20. If that is reduced, there is a reason for it and your eye Doctor should have explained that.
The final component to discuss is the ADD in the prescription. This refers to add power. Most often this is present for patients over the age of 40, as they have embark on to presbyopia. Presbyopia is the event where the muscles responsible for focusing up close have weakened and the lens in the eye loses flexibility. The value is always positive and will usually be somewhere between +1.00 and +2.50. As a patient gets older, the add power will increase to correct their near vision. The add power is summed up with the spherical component of the prescription and takes into consideration astigmatism as well to correct near vision. This is important to understand because I have had patients mistake it for thinking they can get over the counter readers of that power to correct their vision. That may be true for some, but it’s best to ask your eye Doctor if that’s applicable to you!
Prescriptions for vision correction can get quite complicated, but this should have helped you understand what you are looking at a bit more! There are a lot of exceptions to the rules of thumb as well, and your Doctor will be able to clarify any questions you have.
