Understanding the basics of your prescription
It is very common (and perfectly normal!) for each eye to have different corrective needs. That is why your prescription has to specify which eye is in question. The left eye is identified as OS and the right eye as OD (from the Latin Oculus Sinister and Oculus Dexter).
Farsighted, nearsighted, or astigmatic?
The sphere (SPH) refers to the degree of power required for distance or reading glasses. This type of power is distributed evenly all over the lens, and the number will be accompanied by a plus or a minus sign to indicate, respectively, hyperopia (farsightedness) and myopia (nearsightedness). If your prescription needs to correct farsightedness in addition to nearsightedness, it will be marked as ADD, as in “add power”. Pretty straightforward, right?
The cylinder (CYL) refers to the power required to correct astigmatism. Different degrees of power will be assigned to different areas of the lens, along an AXIS. The number associated with the axis refers to the degree of rotation necessary to distribute power with precision.
What is pupillary distance?
The pupillary distance (PD) is the measured distance between pupils. This information is necessary to align the centre of your lenses with the centre of your eyes. If you’d like to learn more about pupillary distance, we’ve got a post for that!
What is prism correction?
Finally, some prescriptions include a power and base direction of a PRISM. To put it plainly, prism correction is used to help people with diplopia, also known as double vision. Diplopia occurs when a misalignment causes light to reach different parts of each retina, which causes the person to see double. This can be corrected by adding a prism to eyeglasses to redirect the light towards the right area of the retinas. The different prism directions are identified as follows: base up (BU), base down (BD), base in (BI) and base out (BO).
What is visual acuity?
Visual acuity (VA) refers to the sharpness and clarity of your vision – it is usually tested with the help of a classic eye chart (it’s called the Snellen chart!) displaying rows of increasingly small letters.
Ultimately, each prescription is unique, and you should always turn to a duly registered eye care professional for specific information regarding your own eye health. We hope we were able to demystify some of the main elements of your prescription, and don't forget to book your next eye exam!