Is 20/40 the same as some diopter number?

Not exactly — the mapping is typical, not fixed. Pupil size, astigmatism and the eye’s own optics shift it, which is why the converter says “roughly.”

What’s a “bad” prescription?

There isn’t one — corrected vision is what matters day to day. But beyond −6.00 the eye is usually longer than average, and that carries retinal considerations worth an annual dilated exam.

Why does astigmatism make it worse?

Cylinder blurs at every distance — it stacks on top of the sphere. Two people at −2.00 see differently if one carries −2.00 of cylinder with it.

My child’s number grows every year. Is that normal?

Common — but not something to just watch. Myopia control (ortho-k, MiSight, atropine, Stellest) slows progression meaningfully. Our Myopia Estimator projects the difference to age 18.

Can high myopia still be corrected to 20/20?

Usually yes — with glasses, contacts or specialty lenses. The number describes the optics, not the eye’s potential; unless disease is present, high myopes typically correct fully.

Medically reviewed by Dr. Bijan Cohenmehr, OD Last reviewed October 24, 2026

What does my prescription mean?

Turn −3.50 into plain English: roughly what the world looks like without your glasses.

Diopters and the 20/xx scale measure different things — lens power versus letter-chart performance — but they’re related enough for a useful translation. Dial in your sphere and read the typical result.

Diopters → 20/xx

Dial in your sphere.

01The two scales

One number is a lens. The other is a chart.

A diopter is optics: the lens power that moves your far point. 20/xx is function: at 20 feet you read what a typical eye reads at xx feet. A −2.00 myope has a far point of 50 cm (everything beyond it blurs) and typically tests near 20/100 uncorrected.

01
Diopter: 1 ÷ focal distance in meters. −2.00 D means the world is sharp out to half a meter, then blurs.
02
20/xx: Snellen acuity: 20/100 means you read at 20 feet what a typical eye reads from 100.
03
Severity bands: Under −3.00 is mild myopia, −3.00 to −6.00 moderate, and beyond −6.00 high, where retinal risks rise and dilated exams matter more.
04
Legal blindness: Defined by corrected vision (20/200 or worse with glasses on), so a −9.00 that corrects to 20/20 is not legal blindness.

Common questions

The numbers, answered.

Anything else, call the office at 310-828-2010.

Not exactly — the mapping is typical, not fixed. Pupil size, astigmatism and the eye’s own optics shift it, which is why the converter says “roughly.”
There isn’t one — corrected vision is what matters day to day. But beyond −6.00 the eye is usually longer than average, and that carries retinal considerations worth an annual dilated exam.
Cylinder blurs at every distance — it stacks on top of the sphere. Two people at −2.00 see differently if one carries −2.00 of cylinder with it.
Common — but not something to just watch. Myopia control (ortho-k, MiSight, atropine, Stellest) slows progression meaningfully. Our Myopia Estimator projects the difference to age 18.
Usually yes — with glasses, contacts or specialty lenses. The number describes the optics, not the eye’s potential; unless disease is present, high myopes typically correct fully.

Go deeper

Numbers are the start. Exams are the answer.

The converter explains the prescription you have; a comprehensive exam with an eye doctor in Los Angeles checks the eye behind it and whether that number should be managed, not just corrected.

Book an Eye Exam Or call 310-828-2010