Home › Learn › How Nearsighted Do You Need to Be for Glasses, and How Often Should You Retest?

GUIDE

How Nearsighted Do You Need to Be for Glasses, and How Often Should You Retest?

Written by Alvin · Manh Long Optical, 147A Le Duan, Cua Nam, Hanoi · Updated 11 September 2026

How nearsighted do you need to be to need glasses?

There's no single number that answers this for everyone. At the same prescription, someone who drives at night will notice it sooner than someone who sits in an office all day. Students reading a whiteboard, office workers looking across a room, people navigating unfamiliar streets, all have different thresholds for when it starts to bother them.

What can be said is: the higher the prescription, the shorter your range of clear vision gets, until squinting or leaning in no longer makes up the difference. Whether to wear glasses, and whether to wear them all day or only for distance, is a conclusion from an in-person test, not a number you read online.

Common thresholds (for reference, not a substitute for testing)

This describes common experience, to help you picture it, not a medical rule:

• Under -1.00: many people still read and get around normally, only slightly blurry at distance in low light. Some choose a pair to keep for driving or the cinema.
• Around -1.00 to -3.00: distance vision starts getting noticeably harder, signs and faces at a distance blur. This is the group that typically starts wearing glasses for distance, and many find it more comfortable to just wear them all the time.
• Above -3.00: the range of clear vision is short, and taking glasses off makes daily tasks difficult. This group usually wears glasses all day.

These thresholds are for reference only. Two people at the same -2.00 can need two different approaches, because astigmatism, the difference between their two eyes, and their job all differ.

Wear them all the time, or only when needed?

With a mild prescription, many people only wear glasses for distance and take them off to read up close. At a higher prescription, wearing them all day is usually more comfortable since the eye doesn't have to keep re-focusing. There's no single right answer, and a refractionist will advise based on your exact prescription and daily habits.

A common misconception: fear that wearing glasses weakens your eyes or speeds up how fast your prescription climbs. Wearing the correct prescription is for seeing clearly and easing eye strain; it isn't what causes a prescription to rise or fall. Prescriptions change with age and individual biology. If yours seems to be changing unusually fast, get retested to update it.

How often should you retest?

Common benchmarks people use:

• Adults with stable vision: roughly every 12 months.
• Children with a still-climbing prescription: more often, roughly every 6 months or as advised at the test.
• First-time wearers, or anyone who just changed prescription: a follow-up check after 2 to 4 weeks to confirm the glasses feel right.

These are starting points, not a fixed schedule. If your prescription is more than 12 months old, retest before cutting new lenses, since your eyes may have changed.

When to test sooner than your schedule says

A few signs suggest getting tested without waiting for your next appointment. These are reasons to get checked, not a diagnosis:

• Distance vision gradually blurring, or needing to squint to see clearly.
• Tired eyes or a headache across the forehead or temples by the end of the day after long screen use.
• Halos or starbursts around headlights or streetlights at night.
• A child sitting close to the TV, tilting their head to watch, or copying schoolwork wrong because they can't read the board clearly.

The following signs mean go to an eye hospital right away, not an optical shop: sudden blurred vision, new double vision, severe eye pain, or a recent eye injury.

What an in-store eye test includes

A refractionist measures refraction on an autorefractor, then confirms it with a trial frame so you can judge which correction actually feels comfortable to look through. The test also checks both eyes, astigmatism and its axis, and pupillary distance (PD) so the lens optical center is placed correctly when it's cut. A full test takes about 20 to 30 minutes.

Once tested, you'll know your current prescription and can choose a lens accordingly. If you're unsure which index to pick, read our guide on choosing lens index by prescription. To understand the numbers on your prescription sheet, see how to read your prescription.

Common questions

How nearsighted do I need to be to need glasses?

There's no single fixed threshold for everyone. Many people start wearing glasses for distance around -1.00, for tasks like driving or reading a whiteboard; below that varies by person. Whether to wear them, and how often, is a conclusion from an in-person test with a refractionist.

How often should I get my eyes retested?

Adults with stable vision usually retest around every 12 months. Children with a still-climbing prescription should test more often, roughly every 6 months or as advised at the test. First-time wearers should follow up after 2 to 4 weeks.

Does wearing the correct prescription make your eyes worse?

Wearing the correct prescription is for seeing clearly and easing eye strain; it isn't what causes a prescription to rise or fall. Prescriptions change with age and individual biology. If yours changes fast, get retested to update it.

Is an eye test free at Manh Long Optical?

Yes. Eye tests in-store are free, even if you don't end up buying glasses, and retesting is free for life. A full test takes about 20 to 30 minutes.