Understanding Your Prescription, Pupillary Distance (PD), Segment Height (SH) & Ocular Height (OCH)

Need Help Understanding All Those Terms?

OD: Stands for “oculus dextrus”, Latin for your right eye.

OS: Stands for “oculus sinister”, Latin for your left eye.

Sphere (SPH): This indicates the lens power, prescribed to correct nearsightedness or farsightedness.

• If the number has a minus sign (–), you are nearsighted.
• If the number has a plus (+), you are farsighted.

Cylinder (CYL): This indicates the lens power for astigmatism. If nothing appears in this column, either you have no astigmatism, or your astigmatism is so slight that it is not necessary to correct it.

• If the number has a minus sign (–), you have a nearsighted astigmatism.
• If the number has a plus sign (+), you have a farsighted astigmatism.

Cylinder power is always after sphere power in an eyeglass prescription.

Axis: The axis defines the location of the cylinder powers on the lens with a number ranging from 1 to 180. Axis is only present if you have a Cylinder correction on your Rx.

PD: Stands for “pupillary distance”, the distance between your pupils.

ADD: The additional value or ADD is commonly used for bifocal or progressive lenses, as well as for reading glasses. Your doctor may write your ADD in the near vision (N.V.) or ADD section.

Prism Correction: This correction is for those who either have exotropia or esotropia. It will cost an additional $10/eye for correction.

Now that you know what it all means, here is how to make sure your prescription is valid.

• If there is a CYLINDER, there must be an AXIS.
• If there is an AXIS, there must be a CYLINDER.
• CYLINDER values must be either both positive or both negative values.
• If it says “SPH” or “SPHERE” for you CYLINDER, that means that there is only a correction for your sphere, not that they have the same value.
• If you order progressive/bifocal lenses, your prescription must have an ADD value.
• If you have an Rx with an “ADD”, but want to order single vision lenses for reading, then you will have to convert your Rx by simply adding the ADD values to your SPH, and that’ll be your new SPH.

Ex: If you have a SPH of +4.00 and an ADD of +2.00, then your new SPH value will be +6.00
Ex: If you have a SPH of -2.50 and an ADD of +1.00, then your new SPH value for that eye will be -1.50.

If you order single vision lenses for reading, and upload a copy of your script, we will make the calculation/conversion for you. For single vision for distance lenses, you can ignore your ADD values and just use the rest of the Rx normally.

Your PD measurement indicates the distance between the centers of your pupils. It is a required measurement that ensures your new lenses are custom aligned over the center of your pupil for proper clarity.

Often times your PD will be indicated on your prescription from your Optometrist. If you don’t have it on hand we can measure it for you in just a few easy steps.

Segment Height, also commonly referred to as Seg Height, is the vertical measurement in millimeters from the bottom of the lens in your frames, to the beginning of the progressive addition on a progressive lens, or the top line of a lined bifocal.

At Lensabl, we request your Seg Height measurements to ensure that your lenses are crafted exactly right for you.

The OC height is the center of the lens vertically from the bottom of the lens to the center of the pupil. OC and PD measurements are needed to properly place the lenses in your frames so that the ocular center (the area that gives you the truest vision) is directly in front of your pupils.