SPH stands for “sphere” on your eye prescription. It’s the number that tells you how much lens power you need to correct nearsightedness or farsightedness, measured in units called diopters. If you’ve just picked up your prescription and are trying to decode the abbreviations, SPH is the most important number on the sheet.
What the SPH Number Tells You
The SPH value represents the overall focusing power your lenses need to bring your vision into sharp focus. Your eye naturally bends light to create an image on your retina, and when that process doesn’t work perfectly, the SPH correction compensates for the difference.
The sign in front of the number is the key detail. A minus sign (−) means you’re nearsighted: you see things clearly up close but distant objects look blurry. A plus sign (+) means you’re farsighted: distance vision is easier, but close-up work is harder. The number itself tells you how strong the correction needs to be. A higher number in either direction means a stronger prescription.
How to Read the Numbers
SPH values are written in increments of 0.25 diopters. You might see something like −2.50 or +1.75 on your prescription. Each eye gets its own SPH value, listed under OD (right eye) and OS (left eye), and the two numbers don’t have to match. It’s common for one eye to need more correction than the other.
For nearsightedness, the ranges break down like this:
- Mild (low myopia): less than −3.00 diopters
- Moderate myopia: −3.00 to −6.00 diopters
- Severe (high myopia): more than −6.00 diopters
If your SPH is something like −1.50, you have a relatively mild prescription. If it’s −7.00, that’s considered high myopia, and it comes with increased long-term risks for conditions like retinal detachment, glaucoma, and cataracts. The International Myopia Institute defines high myopia as anything beyond −6.00 diopters.
SPH vs. CYL and Axis
Your prescription likely has other columns besides SPH, and the most common ones are CYL (cylinder) and Axis. These two numbers work together to correct astigmatism, which is a different kind of focusing problem caused by an irregularly shaped cornea. SPH corrects the overall “spherical” focusing error, while CYL and Axis handle the directional distortion that astigmatism creates.
Not everyone has astigmatism. If your CYL and Axis columns are blank or show zero, your vision issue is purely spherical, and the SPH number is the only correction you need. Many people have some combination of both, which is why all three values appear on the same prescription.
How Your SPH Value Is Determined
During an eye exam, your optometrist or ophthalmologist uses a device called a phoropter, the instrument you look through while they flip between lenses and ask “which is better, one or two?” Each lens swap changes the diopter power in small steps, narrowing down the exact SPH correction that gives you the sharpest vision. Many offices also use an autorefractor beforehand, a machine you stare into that bounces light off your retina to estimate your prescription automatically. The manual “one or two” process then fine-tunes that estimate.
What Changes Over Time
Your SPH value isn’t fixed for life. In children and teenagers, nearsightedness often progresses, meaning the minus number gets larger year over year. This typically stabilizes in the mid-twenties. Farsightedness can also shift, particularly after age 40, when the lens inside the eye gradually loses flexibility. That’s when many people need reading glasses for the first time, even if their distance vision has always been fine. This age-related change is called presbyopia, and it shows up on your prescription as an “ADD” value, which is a separate plus-power boost for close-up vision layered on top of your SPH.
If your SPH has changed significantly between exams, your eye doctor may recommend more frequent checkups. For children with progressing myopia, there are now management strategies aimed at slowing how quickly the number increases, since higher myopia later in life carries greater risk of eye disease.
Glasses vs. Contact Lens Prescriptions
One thing that trips people up: your glasses SPH and your contact lens SPH are not always the same number. Glasses sit about 12 millimeters in front of your eyes, while contacts rest directly on the cornea. That difference in distance changes how light bends before it reaches your eye. For mild prescriptions the difference is negligible, but for SPH values beyond roughly ±4.00 diopters, the contact lens prescription is adjusted (usually a slightly lower number for nearsightedness). This is why you need a separate contact lens fitting and can’t just use your glasses prescription to order contacts.