{
  "slug": "astrophotography-exposure",
  "title": "Astrophotography Exposure Calculator",
  "heading": "Astrophotography Exposure Calculator",
  "category": "other",
  "url": "https://www.revenuelab.fyi/toolbox/astrophotography-exposure",
  "summary": "Max shutter time before stars trail (NPF & 500 rule).",
  "description": "Compare the classic 500 rule against the stricter NPF rule for your focal length, aperture, and sensor pixel pitch.",
  "formula": null,
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=astrophotography-exposure",
  "inputs": [
    {
      "id": "focal",
      "label": "Focal length",
      "kind": "number",
      "hint": null,
      "default": 24,
      "unit": "mm",
      "min": 4,
      "max": null
    },
    {
      "id": "crop",
      "label": "Crop factor",
      "kind": "number",
      "hint": null,
      "default": 1,
      "unit": null,
      "min": 0.5,
      "max": 3
    },
    {
      "id": "aperture",
      "label": "Aperture (f/)",
      "kind": "number",
      "hint": null,
      "default": 2.8,
      "unit": null,
      "min": null,
      "max": null
    },
    {
      "id": "pixelPitch",
      "label": "Pixel pitch",
      "kind": "number",
      "hint": null,
      "default": 4.3,
      "unit": "µm",
      "min": null,
      "max": null
    },
    {
      "id": "declination",
      "label": "Declination",
      "kind": "number",
      "hint": null,
      "default": 0,
      "unit": "°",
      "min": -89,
      "max": 89
    }
  ],
  "outputs": [
    {
      "id": "npf",
      "label": "NPF max exposure (sec)",
      "format": "decimal",
      "hint": null,
      "primary": true
    },
    {
      "id": "rule500",
      "label": "500 rule (sec)",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "eff",
      "label": "Effective focal length (mm)",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "difference",
      "label": "500 rule over-exposure (sec)",
      "format": "decimal",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Focal length: 24 mm",
      "Crop factor: 1",
      "Aperture (f/): 2.8",
      "Pixel pitch: 4.3 µm",
      "Declination: 0 °"
    ],
    "outputs": [
      "NPF max exposure (sec): 9.5",
      "500 rule (sec): 20.8",
      "Effective focal length (mm): 24",
      "500 rule over-exposure (sec): 11.4"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter focal length (mm).",
      "Enter crop factor.",
      "Enter aperture (f/).",
      "Enter pixel pitch (µm).",
      "Enter declination (°).",
      "Read your npf max exposure (sec) on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "focal": 14,
        "crop": 0.6,
        "aperture": 1.68,
        "pixelPitch": 2.5799999999999996,
        "declination": 0
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "focal": 24,
        "crop": 1,
        "aperture": 2.8,
        "pixelPitch": 4.3,
        "declination": 0
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "focal": 38,
        "crop": 1.6,
        "aperture": 4.4799999999999995,
        "pixelPitch": 6.88,
        "declination": 0
      }
    }
  ],
  "limitations": [
    "Estimates assume standard, average conditions — local rules, pricing, and materials vary.",
    "Results are rounded for readability; add a buffer before ordering, booking, or committing.",
    "Double-check anything with a real cost attached against a local quote."
  ],
  "faq": [
    {
      "q": "How does the astrophotography exposure calculator work?",
      "a": "Compare the classic 500 rule against the stricter NPF rule for your focal length, aperture, and sensor pixel pitch."
    },
    {
      "q": "What do I need to enter?",
      "a": "5 values: focal length, crop factor, aperture (f/), pixel pitch, and declination. Each field starts with a sensible default, so you can change one number at a time and watch the result move."
    },
    {
      "q": "What does the npf max exposure (sec) result mean?",
      "a": "It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds."
    },
    {
      "q": "Is this calculator free, and do I need an account?",
      "a": "Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser."
    },
    {
      "q": "How accurate is the result?",
      "a": "It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary."
    },
    {
      "q": "Who is this tool for?",
      "a": "It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/exposure-value",
    "https://www.revenuelab.fyi/toolbox/depth-of-field"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Astrophotography Exposure Calculator (https://www.revenuelab.fyi/toolbox/astrophotography-exposure)"
}