Your vision is described by two main measurements: a visual acuity score (like 20/20) that rates the sharpness of your sight, and a prescription that quantifies exactly how your eyes focus light. You can get clues about your vision from everyday symptoms, but precise numbers require a professional eye exam. Here’s how to make sense of both.
What 20/20 Actually Means
Visual acuity is measured by reading letters on a chart from 20 feet away. The result is written as a fraction. The first number is always 20, your distance from the chart. The second number represents how far away someone with normal vision could stand and still read the same line you’re reading.
If your vision is 20/20, you see at 20 feet what a person with normal eyesight sees at 20 feet. If your vision is 20/40, you need to be at 20 feet to see what a normally sighted person can see from 40 feet. The higher the second number, the blurrier your distance vision. In countries using the metric system, the same test is done at 6 meters, so “normal” vision is written as 6/6 instead of 20/20.
Legal blindness is defined as corrected visual acuity no better than 20/200 in your stronger eye, or a visual field narrowed to 20 degrees or less. That means even with glasses or contacts, the best you can see at 20 feet is what someone with normal vision sees from 200 feet away.
Signs Your Vision May Have Changed
Before you ever sit in an exam chair, your body often signals that something is off. The most common refractive errors each produce distinct patterns. Nearsightedness makes distant objects blurry while close-up vision stays sharp. Farsightedness does the opposite, blurring things nearby. Astigmatism can distort or blur objects at any distance.
Headaches tied to vision problems have a specific profile. They tend to be mild and centered around the forehead, temples, or the eyes themselves. They typically aren’t present when you wake up but build throughout the day, especially after prolonged reading, screen work, or other close-up tasks. Late afternoon and early evening are the most common times for this kind of pain. Other signals include tired eyes, tearing, light sensitivity, and squinting to bring things into focus. If you notice yourself tilting your head or closing one eye to see more clearly, that’s worth paying attention to.
What Happens During an Eye Exam
A comprehensive eye exam does more than read letters off a chart. The standard tool is a Snellen chart, the familiar poster with rows of progressively smaller letters. You cover one eye at a time and read the smallest line you can. The examiner then uses a device called a phoropter, where you look through different lenses and say which one looks clearer, to pinpoint your exact prescription.
Your exam may also include a visual field test, which maps your peripheral vision. In the simplest version, the examiner moves an object from the edge of your vision toward the center and asks when you first see it, repeated in different directions. More advanced automated versions flash tiny lights across a screen while you focus on a central point, pressing a button each time you detect one. This catches blind spots that a standard acuity test would miss entirely.
Depth perception and contrast sensitivity are other dimensions of vision that go beyond simple sharpness. You can have 20/20 acuity and still struggle to judge distances or see well in low-contrast situations like driving at dusk. Contrast sensitivity testing, which measures your ability to distinguish objects from their background, has been suggested as a better indicator of real-world visual quality than acuity alone.
How to Read Your Prescription
Once you’ve been tested, your prescription will contain a few key numbers. Understanding them helps you track changes over time and communicate with any eye care provider.
- OD and OS: OD is your right eye, OS is your left. Some prescriptions use RE and LE instead.
- Sphere (SPH): This is the main lens power needed to correct your vision. A minus sign means you’re nearsighted. A plus sign means you’re farsighted. The larger the number, the stronger the correction you need.
- Cylinder (CYL): This measures astigmatism. If the box is empty, you don’t have any. A higher number means more pronounced astigmatism.
- Axis: A number between 1 and 180 that indicates the angle of your astigmatism on the cornea. It only appears if you have a cylinder value.
A prescription of OD -2.50 -0.75 x 180, for example, means your right eye is moderately nearsighted with mild astigmatism oriented horizontally. Prescriptions for reading glasses or bifocals will also include an “Add” number, which is extra magnifying power for close-up work.
Can You Test Your Vision at Home?
Smartphone apps and printable eye charts can give you a rough idea of where your vision stands, but they have real limitations. One study evaluating a smartphone-based distance vision app found it agreed with the standard Snellen chart only at a moderate level, with a reliability score of 0.61 on a scale where 1.0 is perfect agreement. The app’s accuracy for screening visual impairment was around 93 to 95 percent at its best cutoff point, which sounds high but still means it would misclassify roughly 1 in 15 people.
Home tests are reasonable for noticing a change between exams. If you could read the 20/20 line six months ago and now you’re struggling with 20/30, that’s useful information to bring to your eye doctor. But they can’t measure astigmatism, test your peripheral vision, check eye pressure, or examine the health of your retina. They’re a screening tool, not a replacement.
How Often to Get Your Eyes Checked
The American Optometric Association recommends adults ages 18 through 64 get a comprehensive eye exam at least every two years if they have no symptoms or risk factors. Adults 65 and older should go annually. If you have risk factors like diabetes, a family history of glaucoma, or a history of eye surgery like LASIK, annual exams are recommended regardless of age.
For children, the schedule is tighter. A first exam should happen between 6 and 12 months of age, at least once between ages 3 and 5, and then annually starting before first grade. Kids rarely complain about blurry vision because they assume everyone sees the way they do, so sticking to the schedule matters.
Your vision can shift gradually enough that you don’t notice the change day to day. A prescription that was perfect two years ago may now be leaving you with low-grade headaches or fatigue you’ve attributed to something else. The numbers from each exam create a baseline, making it possible to catch problems like increasing nearsightedness or early signs of disease before they affect your daily life.