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⟳ Vertex Distance Δ Prentice's Rule ◧ Lens Thickness 👁 VA Converter ◱ Slab-Off 🔍 Spectacle Mag.
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Vertex Distance Conversion

Convert spectacle Rx to contact lens plane (or any target vertex distance). Updates live as you type.

Input Rx & Vertex
Result
New Sphere
—
New Cylinder
—
Spherical Equivalent
—
at the target plane
Enter values above — results update live.
F' = F / (1 − d·F)  |  d = vertex shift in meters
Applied independently to sphere & sphere+cyl meridians

Prentice's Rule Calculator

Calculate induced prismatic effect at any point on a lens. Updates live as you type.

Parameters
Result
Prismatic Effect
—
Base direction: —
Enter values above — results update live.
Δ = P × c  |  P = power (D), c = decentration (cm)
Base direction follows standard ophthalmic convention

Lens Thickness Calculator

Frame-based edge thickness at nasal, temporal, superior, and inferior edges — accounts for cylinder axis and OC placement.

Prescription
Base Curve
AUTO
Edit to override. Change sphere to restore auto.
Frame — Boxing System
PD & OC Placement
Edge Thickness at Frame
—
Sup
—
Inf
—
Nas
—
Tem
Nasal
—
 
Temporal
—
 
Superior
—
 
Inferior
—
 
H-Decentration
—
 
OC Vertical Offset
—
 
Values update live as you type.
r=(n−1)/F×1000mm | sag=y²/(2r) | ET=CT+sag_back−sag_front | F(θ)=Fsph+Fcyl·sin²(θ−axis)
👁

Visual Acuity Converter

Convert between Snellen, logMAR, decimal, and percentage visual acuity

Input
logMAR 0.0 = 20/20 | logMAR 1.0 = 20/200 (legal blindness) | logMAR -0.1 = 20/16 (better than normal)
◱

Slab-Off / Differential Prism

Calculate differential vertical prism in downgaze and determine slab-off prescription

Prescriptions
Prentice's Rule: Δ = F × c (cm). Slab-off (bicentric grinding) neutralizes differential vertical prism in the reading position. Rule: apply to the MOST MINUS (or least plus) lens; adds Base-Up prism in the reading zone.
🔍

Spectacle Magnification

Calculate lens magnification from shape and power factors — and estimate aniseikonia between eyes

Single Lens Parameters
Aniseikonia Estimate (optional)
SM = Shape Factor × Power Factor. SF = 1/(1 − t/n × F₁). PF = 1/(1 − d × F). Symptomatic aniseikonia typically ≥ 3–5% image size difference.
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Snellen Hall is an educational and clinical reference tool intended for licensed eye care professionals. All content is provided for informational purposes only and does not constitute medical advice, diagnosis, or a recommendation for treatment. Clinical decisions must be made by a qualified healthcare provider based on individual patient assessment and current clinical guidelines.

While every effort has been made to ensure accuracy and reference peer-reviewed literature, the creators and operators of Snellen Hall make no representations or warranties of any kind, express or implied, regarding the completeness, accuracy, reliability, suitability, or availability of any information contained herein. Clinical data, drug dosages, treatment protocols, and pharmaceutical approvals change frequently — always verify information against current primary literature, package inserts, and institutional guidelines before clinical application.

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Contact lens parameter data is sourced from manufacturer specifications and may not reflect the most current product parameters. Always consult current manufacturer fitting guides and prescribing information.

Private-label equivalents. Store-brand cross-references are a clinical starting point for identification, not an automatic substitution. A private label may differ from its branded counterpart in available powers, axis options, packaging, and product generation, and these arrangements change over time. Verify replacement schedule, BC, DIA, material, and parameter availability, and treat any switch as a refit. Private-label relationships are compiled from published cross-references and manufacturer statements and are not certified by the manufacturers named.

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