What prescription is 20/40 vision?

For a pure myope, 20/40 often corresponds to about −0.50 to −0.75 D of sphere, assuming no significant astigmatism or pathology.

What prescription is 20/200 vision?

For a pure myope, 20/200 often corresponds to about −2.50 to −3.00 D of sphere at a standard test distance, assuming no significant astigmatism or pathology.

What is Egger's rule?

Egger's rule estimates that each Snellen line lost from 20/20 corresponds to roughly −0.25 D of myopia. It applies best in pure myopes with minimal astigmatism.

Can you estimate a prescription from visual acuity alone?

Only roughly for myopia using Egger's rule. Acuity cannot reliably estimate hyperopia, astigmatism, or prescriptions affected by pathology. Always confirm with refraction.

How much anisometropia is clinically significant?

A common practical threshold is around 1.00 D of sphere or 1.50 D of cylinder, with pediatric risk depending on age and adult symptoms possible at smaller differences.

At what age does myopia usually stabilize?

Myopia often slows around ages 18 to 21, but progression into the mid-20s is common. Stability is commonly defined as under 0.50 D change over one year.

At what level is myopia considered pathological?

Pathological myopia is commonly defined as −6.00 D or worse, or axial length over 26 mm, with higher risks of retinal and macular complications.

When should anisometropia be corrected in children?

In children, correction is often recommended at roughly 1.00 D hyperopic anisometropia, 3.00 D myopic anisometropia, or 1.50 D astigmatic anisometropia, depending on age.

Does high astigmatism always indicate keratoconus?

Not always. Stable, regular high astigmatism may be congenital. Progressive, asymmetric, or irregular astigmatism should prompt topography or tomography to rule out ectasia.

What are the signs of keratoconus on refraction?

Scissoring retinoscopy reflex, progressive or oblique astigmatism, asymmetric cylinder between eyes, and reduced BCVA relative to Rx all suggest keratoconus and warrant topography.

Visual Acuity to Prescription Chart and Refractive Error Reference

Estimate prescription from acuity, anisometropia thresholds, and pathological myopia criteria

How to Estimate a Prescription from Visual Acuity

Using uncorrected acuity to estimate myopia

When astigmatism is minimal and accommodation is not driving the result, uncorrected visual acuity often declines in a predictable way as myopia increases. A common chairside approximation, often referred to as Egger's rule, estimates that each Snellen line lost from 20/20 corresponds to roughly −0.25 diopters of spherical myopia in a pure myope.

Treat this as a rough estimate. Test distance, pupil size, media clarity, and ocular pathology can all affect acuity. The visual acuity to prescription chart below is most useful for triage, quick lens estimates, and patient education, not as a substitute for refraction.

Why acuity does not reliably predict hyperopia

Hyperopic patients, especially children and young adults, may accommodate enough to maintain 20/20 acuity despite significant latent hyperopia. Because of that, uncorrected acuity does not reliably estimate the magnitude of hyperopia. When symptoms suggest latent hyperopia, accommodative strain, or accommodative esotropia, cycloplegic retinoscopy or refraction is needed to reveal the true refractive status and to assess amblyopia risk.

How astigmatism affects the acuity-to-prescription relationship

Uncorrected astigmatism degrades acuity differently than spherical myopia. Spherical myopia blurs all meridians, while astigmatism can preserve partial letter recognition along the clearer meridian. As a result, a single Snellen line can represent a range of spherical and cylindrical combinations. Use the visual acuity to prescription chart as a starting point, then confirm with refraction rather than relying on acuity alone in mixed cases.

Estimate Prescription from Visual Acuity

VA	Absolute Myopia Hyperopia	Uncorrected Astigmatism
20/20	0.00	Small
20/30	0.50	1.00
20/40	0.75	1.50
20/60	1.00	2.00
20/80	1.50	3.00
20/120	2.00	4.00
20/200	2.00-3.00	High

When Refractive Error Becomes a Clinical Red Flag

Thresholds for clinically significant refractive error

Refractive error can have structural and functional consequences beyond reduced acuity. In adults, common thresholds used to describe higher risk include myopia worse than −8.00 D, hyperopia greater than +5.00 D, or astigmatism greater than 2.50 D. At these levels, risks related to retinal health, amblyopia history, and binocular vision demands rise, and management may extend beyond standard spectacles.

Amblyopia risk from refractive error in children

In pediatric care, significance is defined by amblyopia risk rather than symptoms. The American Association for Pediatric Ophthalmology and Strabismus (AAPOS) provides age-based criteria that consider anisometropia, astigmatism, and refractive magnitude. Children can have amblyogenic levels of anisometropia or astigmatism before reduced acuity is obvious, so early detection and correction are important for maintaining binocularity.

Pathological myopia versus simple myopia

Distinguish simple myopia from pathological myopia, which is associated with excessive axial elongation and related retinal and macular changes. Eyes with axial length greater than 26 mm or refraction worse than −6.00 D warrant routine dilated fundus examination with careful evaluation of the posterior pole and peripheral retina, regardless of corrected acuity.

Irregular astigmatism and corneal ectasia

When higher astigmatism is paired with a scissoring reflex on retinoscopy, reduced best-corrected acuity, or a refraction that never feels stable, suspect corneal ectasia. These cases warrant corneal topography or tomography to evaluate for keratoconus, pellucid marginal degeneration, or post-surgical ectasia. Early diagnosis supports timely consideration of corneal cross-linking (CXL) and appropriate specialty lens fitting.

Clinically Significant Refractive Error and Red Flags

Error	Isometropia	Anisometropia
Astigmatism	≥2.50D	≥1.50D
Myopia	≥8.00D	≥3.00D
Hyperopia	≥5.00D	≥1.00D

Visual Acuity and Prescription FAQs

What prescription is 20/40 vision?: In a pure myope with otherwise healthy eyes, 20/40 uncorrected acuity often corresponds to roughly −0.50 to −0.75 D of spherical error. However, astigmatism, hyperopia with incomplete accommodation, and ocular pathology can also produce 20/40 acuity, so this estimate is for triage and should be confirmed with refraction.
What prescription is 20/200 vision?: In a pure myope with otherwise healthy eyes, 20/200 uncorrected acuity often corresponds to roughly −2.50 to −3.00 D of spherical error at a standard distance chart. High astigmatism, cataract or other media opacity, and macular disease can also reduce acuity to 20/200 without high myopia, so use this estimate for triage and then confirm with refraction and ocular health evaluation.
What is Egger's rule?: Egger's rule is a chairside approximation that estimates each Snellen line lost from 20/20 corresponds to roughly −0.25 diopters of spherical myopia. It applies best in pure myopes with minimal astigmatism and no significant pathology. For example, 20/30 suggests approximately −0.25 D, 20/40 suggests −0.50 to −0.75 D, and 20/200 suggests −2.50 to −3.00 D. It is a useful triage tool but should not replace refraction.
Can you estimate a prescription from visual acuity alone?: Only as a rough approximation for myopia. Uncorrected acuity can suggest an approximate spherical prescription using Egger's rule, but it cannot reliably estimate hyperopia (because accommodation can mask it), astigmatism (which degrades acuity differently than sphere), or prescriptions in eyes with media opacity or retinal disease. Use the acuity-to-prescription chart for quick estimates, then confirm with a proper refraction.
How much anisometropia is clinically significant?: Anisometropia becomes more likely to cause symptoms or amblyopia risk as the interocular difference increases. A common practical threshold is about 1.00 D of sphere or 1.50 D of cylinder. In children, smaller differences can be amblyogenic depending on age and visual development. In adults, symptoms can occur at lower levels when a long-standing balance is disrupted.
At what age does myopia usually stabilize?: Myopia often slows in late adolescence, commonly around 18 to 21, but progression into the mid-20s is common, especially with high near-work demands and limited outdoor time. A refraction is often considered stable when change is less than 0.50 D over one year, which is useful when discussing elective procedures such as refractive surgery.
At what level is myopia considered pathological?: Pathological myopia is commonly defined around −6.00 D or worse or an axial length greater than 26.0 mm. Above these thresholds, the risk of retinal detachment, myopic maculopathy, and other complications rises compared with emmetropic eyes, so monitoring and patient education become more important.
When should anisometropia be corrected in children?: Pediatric thresholds depend on age and the refractive pattern. Many guidelines recommend correction when anisometropia exceeds roughly 1.00 D of hyperopia, 3.00 D of myopia, or 1.50 D of astigmatism, with lower thresholds in younger children. Use age-based guidance, visual acuity, and binocular findings to determine urgency and follow-up.
Does high astigmatism always indicate keratoconus?: No. High regular astigmatism can be congenital and stable. Concern rises when astigmatism is progressive, markedly asymmetric between eyes, or irregular and cannot be corrected to expected acuity with glasses. Those patterns warrant corneal topography or tomography to evaluate for keratoconus, pellucid marginal degeneration, or other ectatic disease.
What are the signs of keratoconus on refraction?: Suspect keratoconus when you see a scissoring reflex on retinoscopy, progressive or unstable astigmatism (especially against-the-rule or oblique), asymmetric cylinder between eyes, and best-corrected acuity that underperforms relative to the refraction. These findings should prompt corneal topography or tomography. Early detection allows timely referral for corneal cross-linking before significant visual loss.