The spherical value on your eye prescription (usually labeled “SPH” or “Sphere”) is the main corrective power of your lens, measured in units called diopters. It tells you how much correction your eyes need to focus light properly, and whether you’re nearsighted or farsighted. If your prescription has only a spherical value with no other numbers, your vision correction is straightforward. If there are additional values, the sphere is still the foundation everything else builds on.
What the Sphere Number Means
Your eye focuses light by bending it so that it lands precisely on the retina, the light-sensitive layer at the back of your eye. When the shape of your eye is slightly too long or too short, light focuses in front of or behind the retina instead, and your vision blurs. The sphere value represents a lens with uniform curvature in every direction (like the surface of a ball) that shifts that focal point back where it belongs.
The number is always written in diopters, usually in increments of 0.25. A higher number means stronger correction. A prescription of -3.00 bends light more than -1.00, and you’d notice a bigger difference removing those glasses.
Plus vs. Minus: Farsighted or Nearsighted
The sign in front of your sphere number is just as important as the number itself.
- Minus (−): You’re nearsighted (myopic). You see things clearly up close but distant objects are blurry. The lens diverges light slightly to push the focal point back onto your retina.
- Plus (+): You’re farsighted (hyperopic). You see better at a distance but struggle with close-up tasks. The lens converges light to pull the focal point forward onto your retina.
So a sphere of -2.50 means you’re moderately nearsighted, while +1.75 means you’re mildly farsighted. If you see “0.00” or “Plano” in the sphere column for one eye, that eye doesn’t need spherical correction (though it might still need astigmatism correction).
How Strong Is Your Prescription?
For nearsightedness, eye care professionals generally recognize two broad categories. Low myopia covers anything from -0.50 to -5.75 diopters. High myopia is -6.00 diopters or stronger, a threshold established by the International Myopia Institute. High myopia carries a greater risk of certain eye conditions over time, which is why your eye doctor may recommend more frequent checkups if you’re in that range.
For farsightedness, classifications are less standardized, but values under +2.00 are generally considered mild, +2.00 to +5.00 moderate, and anything above +5.00 high.
To put the numbers in everyday terms: someone with an uncorrected sphere of about -0.75 might see roughly 20/40 on a standard eye chart, meaning they need to be at 20 feet to read what someone with perfect vision reads at 40. At around -2.00 to -2.50, uncorrected acuity drops to approximately 20/100 or worse. These conversions are rough because individual eyes vary, but they give you a sense of what the diopter numbers feel like in real life.
Sphere vs. Cylinder: Two Different Problems
Your prescription may also have columns labeled “CYL” (cylinder) and “Axis.” These correct astigmatism, which is a separate issue from what the sphere addresses. While a spherical lens curves the same amount in every direction (picture looking through the side of a perfectly round fishbowl), a cylindrical lens curves along one direction and stays flat in the other (more like looking through the side of a drinking glass laid on its side). Your cornea, the clear front surface of your eye, is supposed to be evenly rounded. In astigmatism, it’s shaped more like a football, so light focuses unevenly. The cylinder value adds extra power along a specific angle (the axis, measured in degrees from 0 to 180) to compensate.
If your CYL and Axis columns are empty or show zero, you have a purely spherical prescription and no astigmatism. Many people have at least a small amount of both.
The “Add” Power for Reading
If you’re over 40, your prescription may include a value labeled “ADD.” This isn’t part of your sphere, but it works alongside it. As the lens inside your eye stiffens with age, it loses the ability to shift focus from far to near. The ADD value is always a plus number (typically between +0.75 and +3.00) that provides extra magnifying power for reading and close work. It gets combined with your sphere to create the bottom portion of bifocals or progressive lenses. So if your distance sphere is -2.00 and your ADD is +2.00, your reading zone effectively has a sphere of 0.00 (plano).
Reading Your Own Prescription
A typical prescription has separate rows for your right eye (labeled OD) and left eye (labeled OS). Each row starts with the sphere value. Here’s what a sample might look like:
- OD: -3.25 (sphere) / -0.75 (cylinder) × 180 (axis)
- OS: -2.75 (sphere) / -1.00 (cylinder) × 175 (axis)
In this example, both eyes are nearsighted (minus sphere). The right eye needs slightly stronger distance correction than the left. Both eyes also have mild astigmatism. It’s completely normal for the two eyes to have different sphere values. Most people aren’t perfectly symmetrical.
One thing worth knowing: a contact lens prescription and an eyeglass prescription aren’t interchangeable, even though both use sphere values in diopters. Contacts sit directly on your eye, while glasses sit about 12 millimeters away, so the effective power changes slightly. Stronger prescriptions show a bigger difference between the two. Your eye care provider will write separate prescriptions for each.