Eyeglasses help the eyes focus light more clearly, allowing patients to see nearby and distant objects with clarity. When you get an eye exam, the optometrist creates a prescription while measuring how the eyes bend light. Using that prescription, an optical lab creates a custom pair of eyeglasses with unique lenses that match each eye. Understanding the numbers and abbreviations on your custom prescription can make the whole process feel less confusing.
The eye uses the cornea and natural lens to bend incoming light. In clear vision, those light rays meet directly on the retina (the light-sensitive tissue at the back of the eye). The retina sends visual information to the brain, enabling sharp, detailed vision.
However, a refractive error occurs when light does not focus directly on the retina. With nearsightedness, the eye focuses light in front of the retina; with farsightedness, the eye focuses light behind the retina. Eyeglasses use different lens shapes and strengths to help move the focus to the right place.
A prescription may look like gibberish upon first glance. Let us dive into what the different abbreviations and numbers mean.
Most prescriptions include separate information for each eye. OD stands for oculus dexter, which means the right eye. OS stands for oculus sinister, which means the left eye. Some prescriptions also use OU; this simply means both eyes. Since each eye can have a different prescription, patients may notice different numbers across the two rows.
The SPH column stands for sphere. This number shows the main amount of lens power needed to correct nearsightedness or farsightedness. Prescription strength is measured in units called diopters.
A minus sign before the sphere number usually shows nearsightedness. For example, a prescription of -2.00 indicates that the eye needs help focusing distant images. A plus sign indicates farsightedness; a prescription of +1.50 means the lens provides additional focusing support for the eye.
Astigmatism occurs when the cornea or natural lens has an uneven curve. Instead of bending light evenly in every direction, the eye focuses light at more than one point. This can cause blurred vision, letter shadows, eye strain, or difficulty seeing clearly at night.
An eyeglass prescription uses CYL and AXIS to describe astigmatism correction. CYL indicates the amount of lens power required for the correct astigmatism, while AXIS indicates the direction, from 1 to 180 degrees, where the correction lies.
The ADD (addition) column often appears on prescriptions for adults who need extra help with close-up vision. This change, called presbyopia, becomes more common with age. The natural lens inside the eye gradually becomes less flexible, making it harder to focus on nearby objects such as books, phones, and menus.
ADD gives the optical lab the extra magnifying power needed for reading. Optometrists may use this number for bifocals, trifocals, progressive lenses, or occupational lenses. In many cases, both eyes have the same ADD value.
Pupillary distance, often called PD, measures the space between the centers of the pupils. This measurement helps align the optical center of each lens with the patient’s natural line of sight. Accurate alignment can make eyeglasses more comfortable and easier to use.
Eyeglasses use carefully measured lens power to help light focus more accurately and improve everyday vision. However, vision does not stay the same over one's lifetime. If you think you need an updated prescription, contact our optometrist today for an appointment.
Request an appointment here: https://brighteyesny.com or call Bright Eyes Optometry at (914) 730-9574 for an appointment in our New Rochelle office.
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