Star Trail / Astro Exposure (500 Rule / NPF Rule) implementation

Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

Record owner

Guth Instruments

Lifecycle

verified-clean

Evidence ID

t4-241

Effect

Computation or source lookup only

Input and validation requirements

A zero focal length produces division by zero in both formulas and must be rejected. In addition, applying crop factor to the NPF formula versus only to the 500 rule changes the result from 5.4285714286 s to 2.7142857143 s, exposing a convention-dependent output.

Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

{
  "inputs": {
    "focal_length_mm": 20,
    "crop_factor": 1,
    "aperture_f_number": 2.8,
    "pixel_pitch_um": 4.14
  },
  "expected_outputs": {
    "500_rule_seconds": 25,
    "npf_rule_seconds": 11.11
  },
  "working": "500 rule: t = 500 / (20 * 1) = 500 / 20 = 25 s. NPF: 35*N = 35*2.8 = 98; 30*p = 30*4.14 = 124.2; numerator = 98 + 124.2 = 222.2; t = 222.2 / 20 = 11.11 s."
}

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche4/tranche4-results.json