A diopter is a unit of optical power, defined as 1 divided by the focal length in metres. It is not an EndMyopia term. Optometry has used it since the late 1800s, and it is the number printed on every eyeglass prescription: -3.00, -5.50, -8.25. A higher negative number means a shorter focal length: for a myope, the eye bends light too strongly for its own length, so distant objects focus in front of the retina instead of on it. EndMyopia did not invent the unit. What it built around it, starting in the site’s own archive from 2012 onward, is a way to track that number without a phoropter: the centimeter measurement, and working rules (the diopter ratio, the diopter gap, the starting-point bands) for reading what a diopter history says about a person’s eyes. Those tools are documented across the site’s own posts, not published research, and this page treats them that way throughout.

The mainstream position, stated plainly first: a diopter number comes from a refraction, the optometrist or ophthalmologist finding the lens power that brings a projected letter chart into sharp focus, then rounding to the nearest quarter diopter. The clinical literature classifies the result by severity, roughly under 3.00 diopters as low myopia, 3.00 to 6.00 as moderate, and above 6.00 as high myopia, a scheme used in the widely cited global prevalence estimate from Holden and colleagues (Holden BA et al., “Global Prevalence of Myopia and High Myopia and Temporal Trends from 2000 through 2050,” Ophthalmology, 2016, doi:10.1016/j.ophtha.2016.01.006). Standard practice, once a diopter number is measured, is to prescribe a lens that fully corrects it and to increase that lens whenever the number rises at a follow-up exam. The number is treated as a fixed property of the eye to be measured and corrected, not a value that responds to how the eye is used day to day.

However, a full-time, fully corrected minus lens removes the one signal, a bit of blur at distance, that the documented EndMyopia trajectory argues the eye responds to; under that standard approach diopter numbers are reported as stable at best and more often rising through the myopic years, and no trial has tested whether a deliberately reduced correction can run that trajectory the other way in adults.

What follows is the EndMyopia reading of the same number: self-reported forum threads, blog posts and case updates, not a controlled study of diopter progression. Where sources disagree with each other, or where a claim is really an estimate dressed up as a rule, this page says so.

What is a diopter?

A diopter (D) measures how strongly a lens, or an eye, bends light. One diopter of power brings parallel light to a focus one metre away; two diopters focus it at half a metre; a -5.00 diopter correction is compensating for an eye whose own optics focus distant light about 20 centimetres in front of where the retina actually sits. The Endmyopia Wiki entry on Diopter covers the same definition and links it forward to the site’s own measurement tools. The one EndMyopia-specific idea layered on top of the clinical definition is the reciprocal relationship between diopters and centimeters: divide 100 by the distance, in centimeters, at which fine print just stops being readable without correction, and the result approximates the myopic diopter value. It is a home approximation of a clinical refraction, not a replacement for one, and the Centimeter measurement wiki entry is the fuller explanation of how it is taken and logged.

What does -3.00 mean in real distance?

Using that same reciprocal relationship, -3.00 diopters corresponds to a blur point around 33 centimeters from the eye without correction, roughly an arm’s length for reading a phone held close, though axial length, pupil size and lighting shift the practical distance a little. This is also why glasses correction is never all-or-nothing in the EndMyopia framing: a lens weaker than the full -3.00 still moves the blur point further out rather than eliminating it, the premise behind reduced, “differential” correction covered on the site’s own how to read an eye prescription hub. An interactive way to see the relationship is the site’s own diopter bubble visualizer, which lets a reader set a diopter value and move a lens-power slider to see the clear zone grow and shrink around it.

Is a higher diopter number worse?

In the clinical sense, yes: a higher number means less unaided acuity at distance and, per the Holden classification above, crosses from low into moderate and then high myopia, categories associated in the same literature with higher long-term risk of retinal complications. The site’s own starting-point posts, organized by band rather than by a single number, make a related but different point: the number by itself does not say much about how the rest of the process goes. Separate posts exist for 2 to 3 diopters, 4 to 5 diopters, and 6 to 8 diopters, each pairing the starting band with case updates from people who began there; the site’s diopter doors carry the same idea into a simple lookup, for example minus-3 or minus-7. One number that does not track cleanly with the sphere value is dim-light acuity: Jake’s own figure, stated without a citation to a measured EndMyopia cohort, is that dim conditions can need roughly half a diopter more correction than daylight to look equally clear.

In dim light, you may need half diopter more to see as clearly as in regular daylight.

https://endmyopia.org/night-blindness-seeing-well-bright-vs-dim-light/

That post also reproduces, at length, an older study from the Journal of the Optical Society of America reporting an average 0.59 diopter myopic shift in dim light across 21 observers, attributed there to chromatic aberration and the eye’s dark focus. The source text names the journal but not a full citation string; this is the Wald and Griffin dim-light refraction study (Wald G, Griffin DR, “The Change in Refractive Power of the Human Eye in Dim and Bright Light,” Journal of the Optical Society of America, 1947, doi:10.1364/JOSA.37.000321).

What is 20/20 in diopters?

There is no fixed conversion, and sources are consistent that treating one as a proxy for the other is a mistake. 20/20 is an acuity measurement, how small a letter a person can resolve at a given distance, while a diopter is a lens power. The same 20/20 result can sit behind very different diopter histories depending on pupil size, lighting, and how recently a person’s correction was reduced. Jake’s caution to newcomers who see a fast early jump in acuity:

A lot of these initial ‘gains’ may be attributed to a combination of over correction of diopters, and reducing your close-up strain, and realizing that you can ‘actively focus’ – rather than just passively seeing things as diopters do all the work.

https://endmyopia.org/newbie-gains-how-to-manage-your-first-diopters/

In other words, an early 20/20 or near-20/20 reading, especially soon after a prescription reduction, is describing the lens and the moment, not a settled new diopter value.

How many diopters is legally blind?

Legal blindness is defined by acuity and visual field, not by diopters: in the United States, best corrected visual acuity of 20/200 or worse in the better eye, or a visual field of 20 degrees or less. There is no diopter threshold in that definition, because a very high myopic correction can still produce normal corrected acuity, while a much lower diopter value paired with other eye disease could meet the legal-blindness bar without high myopia being present. The site’s own material treats the two as separate questions and is skeptical of professionals who conflate a diopter number with a diagnosis, including a post about an optometrist and an ophthalmologist giving wildly different astigmatism readings for the same three-year-old. (source) That post’s underlying point is the same one this section is making: a single diopter reading from one exam is a data point, not a verdict.

Why does my diopter number keep going up?

The mainstream account leans on genetics and, more recently, insufficient outdoor time. The EndMyopia account layers a mechanical one on top: continuous full correction removes the mild-blur stimulus at distance while near work, screens especially, keeps demanding sharp focus up close, and the eye is argued to adapt toward whichever demand it meets most of the day. Jake’s shorthand for that idea, aimed at people who think reducing the number on the lens is itself the fix:

Or of course it could be a Jake’s fault, not spending enough time explaining that it’s resulting stimulus potential that may improve your eyesight, and not the diopter reduction.

https://endmyopia.org/lower-diopters-dont-improve-eyesight/

The video version of that same point is Less Diopters Don’t Improve Your Eyesight! The starting-point posts above make the same distinction repeatedly: a lower number on a lens is a symptom of the underlying stimulus change, not the mechanism itself, and swapping lenses without changing near-work habits is described across the sources as the most common reason people stall.

Can the number go down?

This is the specific claim EndMyopia is built around, and it is the one that matters most to state carefully. The sources are consistent on a pace: roughly a diopter a year for most people, arrived at in quarter-diopter increments every three to four months, never faster by design.

A diopter a year. A quarter diopter every 3-4 months. Milestones that keep you motivated.

https://endmyopia.org/dropping-out-of-double-digit-diopters-12-50-to-9-50-d/

First reduction for normalized can be up to 1 diopter. More, usually doesn’t work out well. Doesn’t matter how much overcorrection or what can be read on Snellen, past one diopter things just don’t go right.

https://endmyopia.org/how-fast-does-my-eyesight-improve-when-can-i-reduce-diopters/

It’s not uncommon for new participants to successfully reduce up to 1 diopter within the first 90 days (just a very general ballpark of possibility). Don’t go over net 1 diopter regardless, even if you feel you could.

https://endmyopia.org/newbie-gains-how-to-manage-your-first-diopters/

What happens when someone ignores that pace is its own recurring genre on the site. One student, Matthew, mapped out what his diopter reductions would have looked like on the recommended four-month schedule against what he actually did, concluded he had moved faster than his adaptation could keep up with, and spent an extended period stuck before working back to the slower pace. (source) None of the case updates in the sources describe sustained change faster than about a diopter a year; several describe the opposite, a fast early drop followed by a long plateau.

What is a diopter gap between the eyes, and why does it matter?

Two related but distinct numbers show up here. The diopter ratio is the difference between the left and right eye’s sphere values; nearly everyone has some dominant eye and some ratio, described in the sources as normal biology rather than a defect. The diopter gap is the distance, in diopters, between a person’s differential (close-up) lens and their normalized (distance) lens.

Main thing to consider is that you likely don’t want it to be less than one diopter, and very likely no more than 2 diopters.

https://endmyopia.org/pro-topic-managing-your-maximum-diopter-gap/

On the ratio specifically, the sources are firm that it should not be changed casually or early:

Don’t mess with the ratio. Until you’re well into several successful bifocal, equal diopter reductions, that’s a no-no.

https://endmyopia.org/the-diopter-ratio-trap-dont-favor-one-eye/

The recommended way to decide whether and when to equalize a stubborn ratio is to look at each eye’s own centimeter trend rather than guess from the prescription:

The best way to tell is based on your centimeter changes on previous reductions.

https://endmyopia.org/pro-topic-nick-is-time-to-equalize-diopters/

A follow-up post on fixing an already large ratio treats it as a case-by-case problem rather than a formula, and points back to the same centimeter log as the starting point for figuring out what happened and when. (source) A DIY method some students use to work the weaker eye specifically is light patching, wax paper or a translucent cover over the dominant lens for short stretches, described in the sources as a small, occasional supplement rather than a core practice. (source) Where more than a diopter of astigmatism correction is involved, the sources treat it as its own, harder problem, covered in more depth on the site’s own astigmatism hub.

How do I measure my own diopters at home?

The starting point across the sources is the centimeter measurement described above: find the closest distance, without correction, at which small print stops being readable, and convert with the 100-divided-by-centimeters approximation. Jake’s framing of why that measurement, not a guessed diopter number, is the thing worth tracking:

Diopters are the result of a process of discovery.

https://endmyopia.org/dont-want-give-diopter-advice/

Your first question should not be about diopters at all.

https://endmyopia.org/how-many-diopters-do-you-need/

The video companion to that second post walks through the same reasoning: Newbie Mistakes: How Many Diopters Do I Need? Practically, that means logging the centimeter number regularly, at the same time of day and lighting where possible, and letting the diopter value fall out of that trend rather than picking a target number first and working backward to it. The site’s own eyesight numbers tool is built to make that lookup, and the reciprocal conversion, quick to run without a calculator.

Measure, don’t believe

Nothing above is worth taking on faith, including the one-diopter-a-year figure and the centimeter formula this page uses to explain it. The sources consistently point back to the same starting move: measure the closest distance at which fine print stops being readable without correction, log it, and re-check it every few weeks rather than trusting how sharp things feel on any given day.

To turn that measurement into an actual diopter estimate rather than a guess, the eyesight numbers tool runs the conversion and shows the matching band, low, moderate or high myopia, for what was just measured.

Further reading

• Diopter, Endmyopia Wiki
• Centimeter measurement, Endmyopia Wiki
• Prescription, Endmyopia Wiki
• Starting Point: -2 To -3 Diopters
• Starting Point: -4 To -5 Diopters
• Starting Point: -6 To -8 Diopters
• The Diopter Ratio Trap: Don’t Favor One Eye
• Pro Topic: Managing Your Maximum Diopter Gap
• How Fast Does My Eyesight Improve? When Can I Reduce Diopters?
• Why You Don’t Want To Give Diopter Advice
• How To Read Your Eye Prescription
• What Is Active Focus?
• Astigmatism, explained