SPH stands for “sphere” on your glasses prescription, and it tells you how much lens power you need to correct your vision. The number is measured in diopters, and it can be positive or negative depending on whether you’re farsighted or nearsighted. It’s the most fundamental number on your prescription, and understanding it helps you make sense of everything else written on that slip of paper.
What the SPH Number Means
The SPH value represents the amount of correction your eye needs to focus light properly onto your retina. A minus sign (−) in front of the number means you’re nearsighted, meaning you see things clearly up close but struggle with distance. A plus sign (+) means you’re farsighted, meaning distance vision comes more naturally but close-up tasks are harder.
The number itself tells you how strong the correction is. A prescription of −1.00 is a mild correction for nearsightedness, while −9.00 is a strong one. The farther from zero the number gets in either direction, the more your eye’s natural focusing ability differs from ideal, and the more powerful your lenses need to be.
If you see “PL” or “Plano” in the SPH column, it means that eye has no spherical refractive error and doesn’t need correction for distance or near focus. You might still have a value in the other columns for astigmatism, though.
How SPH Differs From CYL and Axis
Your prescription likely has three main columns: SPH, CYL, and Axis. SPH corrects for the overall focusing power of your eye, treating it as though the eye is shaped like a round ball. CYL (cylinder) corrects for astigmatism, which happens when your eye is shaped more like a football than a basketball, causing light to focus unevenly. Axis is a number between 1 and 180 that tells the lab which angle to orient the astigmatism correction.
If your CYL and Axis columns are blank, or if you see “DS” (diopter sphere) written there, you don’t have astigmatism and your lenses only need the spherical correction. Many people have some combination of both.
Mild, Moderate, and High Prescriptions
For nearsightedness, anything from −0.50 to −6.00 is classified as low to moderate myopia. Once you hit −6.00 or stronger (meaning −6.50, −7.00, and beyond), it’s considered high myopia. High myopia carries a greater risk of certain eye conditions over time, so your eye doctor may recommend more frequent exams.
There isn’t as widely standardized a classification system for farsightedness, but generally anything above +2.00 starts to be noticeable in daily life, and values above +5.00 are considered significant.
How Your Eye Doctor Finds Your SPH
During an eye exam, your optometrist determines your SPH value through a process called refraction. This is the part where you look through the phoropter (the device with all the dials and lenses) and answer “which is better, one or two?” as different lenses click into place.
The process is more systematic than it feels. Your doctor starts by placing small test lenses in front of your eye, typically in increments of 0.25 diopters, and asks which option makes the letters on the chart clearer and sharper. They’ll often “fog” your vision first by adding extra plus power, then gradually reduce it until you reach the clearest image. A red-green comparison test helps fine-tune the result: when letters on both the red and green backgrounds look equally clear, the sphere is dialed in. After any astigmatism correction is set, the sphere gets refined one final time.
How SPH Affects Your Lenses
The strength of your SPH value directly impacts the thickness and weight of your lenses. Nearsighted lenses are thinner in the center and thicker at the edges, while farsighted lenses are the opposite, thicker in the middle and thinner at the edges. Either way, a stronger prescription means more material.
If your SPH is above +2.00 or below −2.00, your optician may recommend high-index lenses. These use a denser material that bends light more efficiently, so the lens doesn’t need to be as thick. Standard plastic lenses have a refractive index of 1.50, while high-index options range from 1.53 to 1.74. At the top end, a 1.74 index lens can be up to 50 percent thinner than a standard plastic lens with the same prescription. The tradeoff is cost: high-index lenses are more expensive, and the difference in thickness is less dramatic for milder prescriptions.
When Your SPH Typically Stabilizes
If you’ve noticed your prescription changing at every exam, that’s normal, especially through your teens and early twenties. For most people, the SPH value stabilizes around age 25 and stays roughly the same until the early 40s. At that point, a different issue kicks in: presbyopia, the gradual loss of close-up focusing ability that comes with aging. Presbyopia is separate from your SPH and is corrected with an “ADD” value on your prescription, which is why many people over 40 end up with bifocals or progressive lenses even if their distance prescription hasn’t changed.
Children whose SPH falls between +0.75 and −0.50 may be flagged as “pre-myopic,” meaning their current prescription is borderline but their age and risk factors suggest they’re likely to develop nearsightedness. Early intervention strategies can sometimes slow the progression.