{
  "schemaVersion": 1,
  "slug": "nd-filter-calculator",
  "category": "photography",
  "kind": "implementation",
  "independentValidation": false,
  "implementationHash": "e5af0f58f6844505ec6492952fee3473187ae0a8182b00e15e549cd586c84e7c",
  "provenance": "Generated from the calculation contract used by the published page. Publication is not external validation.",
  "sourceFiles": [
    "data/calculators/nd-filter-calculator.json",
    "src/engine.js"
  ],
  "runtimeSource": "/sources/engine.txt",
  "runtimeHash": "5c888d8898afbdee36267bb9977829e1e2ac568f6677c8ee87469c139864645c",
  "scope": "The compute function records implemented equations, constants, lookup tables and custom rules. Inputs record defaults and allowed values; outputs record precision and units. Defaults are examples, not recommendations. General background references are not evidence for this specific algorithm.",
  "publishedFormula": {
    "formula": "t₂ = t₁ × 2^s",
    "vars": [
      [
        "t₂",
        "new exposure time"
      ],
      [
        "t₁",
        "base shutter time"
      ],
      [
        "s",
        "ND filter strength, stops"
      ]
    ],
    "note": "The factor 2^s doubles exposure per stop; display rule: ≥1 s shown rounded to 0.1 s, otherwise as 1/round(1/t) s."
  },
  "externalEvidence": {
    "reviewedAt": "2026-10-12",
    "sources": [
      {
        "name": "Nikon — Neutral-density filters and exposure stops",
        "url": "https://www.nikonusa.com/learn-and-explore/c/tips-and-techniques/the-joy-of-long-exposure-photography"
      }
    ],
    "referenceRole": "context",
    "implementationHash": "e5af0f58f6844505ec6492952fee3473187ae0a8182b00e15e549cd586c84e7c",
    "basis": "t₂ = t₁ × 2^s",
    "limitations": "The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions."
  },
  "contract": {
    "compute": "const factor = Math.pow(2, filterStops);\nconst neu = shutterSeconds*factor;\nconst label = (s)=>{\n  if(!(s>0)) return '\\u2014';\n  if(s >= 1) return (Math.round(s*10)/10) + ' s';\n  const den = Math.round(1/s);\n  return '1/' + den + ' s';\n};\nreturn { newSeconds: neu, increaseFactor: factor, stops: filterStops, newShutterText: label(neu), baseShutterText: label(shutterSeconds) };",
    "inputs": [
      {
        "name": "shutterSeconds",
        "type": "number",
        "default": 0.002,
        "min": 0.00001,
        "max": 3600,
        "step": 0.0001,
        "precision": 5
      },
      {
        "name": "filterStops",
        "type": "number",
        "default": 10,
        "min": 1,
        "max": 20,
        "step": 0.5,
        "slider": true
      }
    ],
    "outputs": [
      {
        "name": "newSeconds",
        "precision": 2,
        "unit": "s"
      },
      {
        "name": "increaseFactor",
        "precision": 0,
        "unit": "×"
      },
      {
        "name": "stops",
        "precision": 1,
        "unit": "stops"
      },
      {
        "name": "newShutterText",
        "precision": 0,
        "unit": "",
        "type": "text"
      },
      {
        "name": "baseShutterText",
        "precision": 0,
        "unit": "",
        "type": "text"
      }
    ],
    "interpret": null
  }
}