While a certified optometrist is the best person to read your prescription, it can be helpful to understand how they do it. Think of it like a sneak peek behind the curtain!
Firstly, it’s important to know that prescriptions are written in a specific format, with numbers and abbreviations that refer to different aspects of your eyesight. These numbers typically include things like the strength of your prescription (measured in diopters), your astigmatism, and the distance you need to see clearly (e.g., for reading or driving).
When you visit an optometrist, they will use a variety of tests and tools to measure your eyesight and determine your prescription. This might involve looking at an eye chart, using a phoropter (a device with a series of lenses that can be adjusted to find the optimal prescription), and other tests to check for color vision and depth perception.
Once the optometrist has determined your prescription, they will typically write it down on a prescription pad or enter it into a computer system. This prescription can then be used to order glasses or contacts or to inform any other necessary treatments.
While it’s always best to seek the expertise of a certified optometrist when it comes to your eyesight, having a basic understanding of how prescriptions work can help you understand your vision and make informed decisions about your eye health.
Common Eyeglass Prescriptions Guide
OD & OS:
OD and OS are abbreviations used in prescription eyeglasses or contact lenses to specify the prescription for the right and left eyes, respectively.
The OD on your optical prescription refers to “oculus dexter,” Latin for the “right eye.” The right eye’s prescription is always listed first. And the OS prescription for the left eye is always listed second on the prescription.
For example, if a prescription says OD -2.00 and OS -1.75, the right eye needs a lens with a power of -2.00 diopters, and the left eye needs a lens with a power of -1.75 diopters.
It’s important to note that the prescription for each eye may be different based on the individual’s vision needs, and the prescription may also include additional information such as the cylinder (CYL) and axis for astigmatism correction, the sphere (SPH) for nearsightedness or farsightedness correction, and the prism for correcting eye alignment issues.
SPH (or sph.) is an abbreviation for sphere, which measures the power of lenses needed to focus light correctly into the eyes. It indicates how strong or weak a lens needs to be to correct vision issues such as nearsightedness or farsightedness.
Here are the steps to read an SPH prescription:
- Look for the SPH number: The SPH number will be written on the prescription and typically ranges from -20.00 to +20.00 diopters.
- Determine the type of refractive error: A negative SPH number indicates that the person has nearsightedness, which means they have difficulty seeing distant objects clearly. A positive SPH number indicates that the person has farsightedness, which means they have difficulty seeing close-up objects clearly.
- Interpret the SPH number: The higher the SPH number, the greater the amount of spherical power needed to correct the refractive error. For example, a prescription with an SPH of -2.50 diopters indicates that the person has moderate nearsightedness and requires a lens with a negative spherical power of 2.50 diopters to correct their vision.
Cylinder (or cyl) is an indication of astigmatism, which is when the cornea and/or lens of an eye are not perfectly spherical. This measurement tells how much correction needs to be done on the curvature of the lens when it comes to focusing light.
Here are the steps to read a CYL prescription:
- Look for the CYL number: The CYL number will be written on the prescription and typically ranges from -0.25 to -4.00 diopters.
- Determine the amount of astigmatism: The CYL number indicates the amount of cylindrical power needed to correct astigmatism. This power is added to the spherical power (SPH) to create a prescription that corrects spherical and cylindrical errors.
- Interpret the CYL number: A negative CYL number indicates that the astigmatism is oriented vertically, while a positive CYL number suggests that the astigmatism is oriented horizontally. The higher the CYL number, the greater the cylindrical power needed to correct astigmatism.
An axis prescription is a component of a prescription for eyeglasses or contact lenses that are used to correct astigmatism, a common condition where the cornea of the eye is not perfectly round, causing blurred or distorted vision. The axis is a number between 0 and 180, indicating the astigmatism angle.
Here are the steps to read an axis prescription:
- Look for the axis number: The axis number will be written on the prescription and typically ranges from 0 to 180 degrees.
- Determine the orientation of the axis: The axis is always oriented horizontally or vertically. If the axis number is 90 degrees, the astigmatism is vertical. If the axis number is 180 degrees, the astigmatism is horizontal.
- Interpret the axis number: The axis number indicates the angle of astigmatism in degrees. It is used to determine the orientation of the cylindrical lens needed to correct astigmatism. For example, an axis number of 45 degrees means that the cylindrical lens needs to be oriented at a 45-degree angle to correct astigmatism.
Add (or ADD) stands for ‘addition’, indicating that additional power needs to be added at certain points on lenses due to presbyopia (the inability of eyes to focus on close objects). This measurement helps opticians determine how much additional focusing power needs to be added onto a prescription lens in order for you to have a better near vision.
Here are the steps to read an ADD prescription:
- Look for the ADD number: The ADD number will be written on the prescription and typically ranges from +0.25 to +3.50 diopters.
- Determine the power needed for close-up vision: The ADD number indicates the additional magnifying power required to correct presbyopia. This power is added to the distance prescription and is used to create a bifocal or progressive lens that allows the wearer to see both near and far.
- Interpret the ADD number: The higher the ADD number, the greater the magnifying power needed to correct presbyopia. For example, an ADD number of +2.00 indicates that the wearer needs an additional magnifying power of 2.00 diopters to see close-up objects.
Prism is used when a person’s eyes do not work together properly because one eye may turn inward or outward compared with the other. Optometrists measure this misalignment between eyes by measuring its strength and direction using prism measurements (measured in diopters) before prescribing prism glasses that can help align both eyes correctly, and vision will appear normal again.
Here are the steps to read a prism prescription:
- Look for the prism number: The prism number will be written on the prescription and is typically expressed in prism diopters (PD). It can be a horizontal (base-out or base-in) or vertical (base-up or base-down) prism.
- Determine the prism’s direction and magnitude: The prism’s path is indicated by the base, which is either base-in (towards the nose) or base-out (away from the nose) for horizontal prisms and base-up or base-down for vertical prisms. The magnitude of the prism is indicated by the prism diopter (PD) value.
- Interpret the prism prescription: For example, a prescription with a prism of 2 PD base-out means that the lens is designed to shift the visual image outward by two prism diopters to correct a horizontal misalignment of the eyes.
It is important to note that each can be one or two components of a complete eyeglasses or contact lens prescription. It is always recommended to consult with an optometrist or ophthalmologist to determine the best treatment for your specific eye condition.