Vision Basics

Understanding Prescriptions

In order to understand the relative ease of a particular correction and the likelihood that a patient will qualify for surgery, it is helpful to learn about eyeglass and contact lens prescriptions.

The basic format for a prescription is: +/– sphere +/– cylinder x axis.

Here are some examples:

-3.25 -1.75 x 176
+1.25 -0.50 x 90
+2.00 +1.50 x 87
Plano +0.50 x 66
-6.50 sph.

Sphere refers to the amount of nearsightedness (myopia) or farsightedness (hyperopia). If the sphere number is negative, the patient is nearsighted. If it is positive, he or she is farsighted. Sphere numbers of zero are called "plano" or abbreviated "PL".

Cylinder refers to the amount of astigmatism. If no astigmatism is present, then the prescription will only have a number for sphere. Cylinder can be expressed as either a positive or negative number. Most surgeons use positive numbers, known as "plus cylinder format", while most non-surgeons use negative numbers, know as “minus cylinder format”.

Prescriptions in minus cylinder need to be converted into plus cylinder in order to accurately read the following tables. To convert a prescription, simply add the sphere and cylinder to obtain the converted sphere and change the cylinder to a positive number. If the axis is =>90, then subtract 90 for the new axis, otherwise add 90.

Prescription Conversion Example
original prescription in minus cylinder = -3.25 -1.75 x 176
-3.25 + (-1.75) = -5.00 = new sphere
+1.75 = new cylinder
176 - 90 = 86 = new axis
-5.00 +1.75 x 86 = converted prescription in plus cylinder


Axis refers to the direction on the clock hour of the astigmatism. It is not necessary to understand anything about the axis number when contemplating laser vision correction, nor is it necessary to understand any of the following terms that are occasionally found on a prescription: "Add", "Prism", "Base Curve", "Diameter" and "Pupillary Distance (PD)". However, if a prescription contains the word “Prism”, this finding should be mentioned to the doctor, as it may indicate an eye alignment problem that makes laser vision correction inadvisable.

Here are some prescription examples:

Sample Prescription #1:  –3.75 –0.50 x 180

Because the sphere is negative, this individual is nearsighted. Notice, however, that the cylinder number is in minus format (i.e., a negative number). When converted to plus cylinder format, the resultant prescription is –4.25 +0.50 x 90.

Sample Prescription #2: +2.25 sph, add +1.25

Because the sphere is positive, this individual is farsighted. The "add" number is used when a bifocal lens is needed to help with reading. The "add" number is not relevant for our purposes. Notice there is no number for cylinder, so this individual doesn't have astigmatism.

Sample #3: –2.25–1.50x110, BC=8.8 Dia=14.0

This is a contact lens prescription. The terms BC (base curve) and Dia (diameter) refer to the size of the lens. It is not necessary to understand these terms for our purposes. When converted to plus cylinder format, the prescription reads –3.75 +1.50 x 20.

With the basics in mind, let’s take a look at what prescription numbers mean with respect to the relative ease of a particular correction and the likelihood of qualifying for surgery.

The basic concept is as expected. Eyes with lower prescription numbers are easier to correct and less likely to run into issues that make surgery inadvisable.

But, what is a low prescription?

Though each surgeon has his or her own criteria, here are some general guidelines. Please note that prescriptions with a negative number for astigmatism (cylinder) must be converted to plus cylinder format before using the tables below.

Myopia (Nearsightedness)

0.00 to -3.00low
-3.01 to -6.00moderate
-6.01 to -9.00high
-9.01 to -12.00very high

Hyperopia (Farsightedness)

0.00 to +2.00low to moderate
+2.01 to +4.00moderate to very high
+4.01 to +6.00very high to extreme


0.00 to +1.50low to moderate
+1.51 to +3.00moderate to high
+3.01 to +4.50high to very high
+4.51 to +6.00very high to extreme

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