Whether your eye prescription is “bad” depends entirely on how far your numbers are from zero. A prescription between -0.25 and -2.00 is mild and extremely common, while anything beyond -6.00 or +5.00 is considered high and worth paying closer attention to. Most people fall somewhere in the mild-to-moderate range, and even prescriptions that seem strong are usually correctable with glasses, contacts, or surgery.
To figure out where you stand, you need to understand what the numbers on your prescription actually mean and what thresholds eye doctors use to separate “no big deal” from “keep a close eye on this.”
How to Read Your Prescription
Your prescription has a few key columns. The most important one is SPH (sphere), which represents the main correction your eyes need. This number is measured in diopters. A negative number means you’re nearsighted (you struggle to see far away), and a positive number means you’re farsighted (close-up vision is harder). The farther this number is from zero in either direction, the stronger your prescription.
If you have astigmatism, your prescription will also include CYL (cylinder) and Axis. The cylinder number reflects how much correction is needed for the irregular curvature of your eye, while the axis (a number from 0 to 180) simply tells the lens maker where to position that correction. A small cylinder value like -0.50 is barely noticeable. A larger one like -2.50 or beyond is significant.
Some prescriptions also include a prism measurement, which is less common. Prism correction is used when your eyes don’t align properly, causing double vision. The prism bends light so both eyes receive the image in the right spot, and the number (measured in prism diopters) indicates how misaligned your eyes are.
Nearsightedness: Mild, Moderate, and High
The American Academy of Ophthalmology breaks nearsightedness into three levels based on the sphere value:
- Mild (low myopia): Less than -3.00 diopters
- Moderate: -3.00 to -6.00 diopters
- Severe (high myopia): Beyond -6.00 diopters
If your prescription is -1.50, you’re on the mild end. You probably can’t read a road sign without correction, but your eyes are in a very common range and there’s nothing to worry about. A prescription of -4.00 is moderate. Everything more than a couple feet away is blurry without correction, but it’s still a routine prescription that any pair of glasses or contacts handles easily.
Once you cross -6.00, your prescription is classified as high myopia. This doesn’t mean your vision can’t be corrected, but it does mean your eyes have physically elongated more than average, which creates some long-term considerations worth knowing about.
Farsightedness and Astigmatism Ranges
Farsightedness follows a similar scale on the positive side:
- Low: +2.00 or less
- Moderate: +2.25 to +5.00
- High: +5.25 or more
For astigmatism, the cylinder value on your prescription tells you the severity:
- Insignificant: -0.25 to -0.50
- Low: -0.75 to -1.00
- Moderate: -1.25 to -2.25
- High: -2.50 or beyond
Many people have a small amount of astigmatism and don’t even realize it. If your cylinder is -0.50, some doctors won’t even bother correcting for it. Once it reaches -1.25 or higher, you’ll likely notice that vision feels distorted or “streaky” without correction, especially at night.
When a High Prescription Affects Eye Health
A strong prescription is more than an inconvenience. In high myopia, the eyeball is physically longer than normal, which stretches the retina thinner over time. This creates real, measurable risks. Compared to someone with perfect vision, a person with mild myopia (up to -3.00) has about 4 times the risk of retinal detachment. Beyond -3.00, that risk jumps to 10 times higher. For people with high myopia past -5.00, the lifetime risk of retinal detachment is roughly 20 times higher than average.
High myopia can also lead to a condition where the stretched retina begins to degenerate, causing thinning and atrophy of the tissue at the back of the eye. This is what doctors call pathological myopia, and it’s defined not just by the prescription number but by visible changes to the retina. Not everyone with high myopia develops these changes, but the risk is significant enough that the American Academy of Ophthalmology now recommends annual dilated eye exams for anyone with a prescription of -6.00 or stronger.
If your prescription is in the moderate or mild range, these risks are much lower. You should still get regular eye exams, but there’s no special cause for concern.
What About Legal Blindness?
A high prescription number alone does not make you legally blind. Legal blindness is defined as having 20/200 vision or worse, meaning you need to be 20 feet from something a person with normal sight can see at 200 feet. The key detail: this measurement is taken with your best correction. If your glasses or contacts bring you to 20/40 or 20/20, you’re not legally blind regardless of how strong your prescription is.
Someone with -10.00 might see terribly without glasses but perfectly fine with them. Legal blindness only applies when corrective lenses can’t get you to that 20/200 threshold.
Can Surgery Fix a Strong Prescription?
LASIK and PRK can correct a wide range of prescriptions. Current laser systems are approved to treat up to -12.00 diopters of nearsightedness, up to +6.00 of farsightedness, and up to 6.00 diopters of astigmatism. That covers the vast majority of people, including many with high prescriptions.
However, having a very high prescription can make you a less ideal candidate. Correcting a large amount of myopia requires removing more corneal tissue, and if your corneas are too thin, the procedure may not be safe. Your eye doctor will measure corneal thickness and other factors to determine whether you qualify. For people whose prescriptions exceed laser limits, implantable lenses are another option that can handle even stronger corrections.
Putting Your Numbers in Context
If your sphere is between -1.00 and -3.00, you have a common, mild prescription. You need glasses for driving and movie theaters, but your eye health risk profile is essentially the same as someone with perfect vision.
Between -3.00 and -6.00, your prescription is moderate. Glasses or contacts are a daily necessity, and you should keep up with regular eye exams, but this is still a very manageable range. Most people in this zone are good candidates for laser surgery if they want it.
Beyond -6.00 (or beyond +5.00 for farsightedness), your prescription is genuinely high. Your vision is fully correctable in most cases, but your eyes need more monitoring. Annual dilated exams become important, and you should know the warning signs of retinal problems: sudden flashes of light, a shower of new floaters, or a shadow creeping across your vision. These warrant a same-day visit to an eye doctor.
For astigmatism, anything under -1.00 in the cylinder column is minor. Over -2.50 is considered high, but even strong astigmatism is routinely corrected with toric contact lenses or glasses. It just means your lenses need to be more precisely positioned.