Description
Scale an explicitly calibrated Snellen chart fraction for a supplied test distance.
Optical calculations distinguish lens power in diopters, prism notation, angular axes, geometric distances, and visual-acuity reporting scales. Sign conventions and length units are especially important when a formula combines power with decentration or vertex distance. A supplied model term is separate from a validated visual test or prescription.
Scale an explicitly calibrated Snellen chart fraction for a supplied test distance.
When to use this tool
Use Optometric Chart Distance Acuity Factor when you need this explicit optical, chart, frame, axis, or reference conversion with compatible supplied units.
How to use the tool and how it works
- Enter testDistance, calibrationDistance, chartDenominator, numerator. Use the units and conventions below.
- Run the tool. Correct any format, range, or cross-field input error before interpreting the output.
- Read adjustedDenominator, decimalAcuity. Use the output convention below.
For positive testDistance, calibrationDistance, chartDenominator, and numerator return adjustedDenominator=chartDenominator*calibrationDistance/testDistance and decimalAcuity=numerator*testDistance/(chartDenominator*calibrationDistance).1
| Input field | Meaning and units | Accepted input |
|---|---|---|
| testDistance | Test Distance | Required. Finite number; strictly above 0. |
| calibrationDistance | Calibration Distance | Required. Finite number; strictly above 0. |
| chartDenominator | Chart Denominator | Required. Finite number; strictly above 0. |
| numerator | Positive acuity-notation numerator on the chartDenominator basis. | Required. Finite number; strictly above 0. |
| Output field | Interpretation | Returned shape |
|---|---|---|
| adjustedDenominator | Adjusted Denominator | Required. Finite number. |
| decimalAcuity | Decimal Acuity | Required. Finite number. |
{
"input": {
"testDistance": 3,
"calibrationDistance": 6,
"chartDenominator": 12,
"numerator": 6
},
"output": {
"adjustedDenominator": 24,
"decimalAcuity": 0.25
}
}Limitations and assumptions
- The distance ratio needs matching units and numerator/denominator share one chart convention. The arithmetic rescales supplied chart notation without administering a test, handling nonstandard scores, or diagnosing vision.
- Inputs must satisfy both the field types and the cross-field conditions above. Invalid input causes an error; a successful false flag or documented placeholder is a different result.
- Numeric calculations use binary floating point unless exact decimal-integer arithmetic is explicitly stated. No hidden measurement tolerance or decimal rounding is added; nonfinite or out-of-contract results are rejected. Numeric signed-zero results are exported as zero.
Alternative or Complementary methods
Use the appropriate complete optical or measurement model when a scalar term is insufficient. Verify power, axis, length, notation, and reference conventions before applying the result within a broader assessment.
References
Similar or alternative tools
- Snellen Decimal Logmar Source Crosswalk
Numerical Snellen fraction, decimal and logMAR crosswalk, excluding counting-fingers or other non-Snellen categories and clinical interpretation.
- Ophthalmic Chart Distance Letter Height Factor
Geometric equal-angular-size chart scaling, without asserting clinical equivalence between chart designs.