Source: https://guthlabs.ai/docs/instruments/nd-filter-stop-exposure-time-calculator/implementation/

instrument implementation / implementation-contract

# ND Filter Stop / Exposure Time Calculator implementation

A source-bound implementation contract for ND Filter Stop / Exposure Time Calculator; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## 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**: t4-193

**Effect**: Computation or source lookup only

## Input and validation requirements

The specification admits filter_factor = 0, for which log2(0) and the derived stop count are undefined; the calculator must reject it. ND1000 also produces materially different results depending on whether it is interpreted as exact factor 1000 or the common rounded value of 10 stops.

## 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": {
    "baseTimeSeconds": 0.008,
    "stops": 3
  },
  "expected_outputs": {
    "newTimeSeconds": 0.064,
    "humanReadable": "64 ms (approximately 1/15.6 s)"
  },
  "working": "2^3 = 8. 0.008 s × 8 = 0.064 s. 0.064 s × 1000 = 64 ms. Reciprocal shutter speed: 1 ÷ 0.064 = 15.625, so approximately 1/15.6 s."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/nd-filter-stop-exposure-time-calculator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/nd-filter-stop-exposure-time-calculator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/nd-filter-stop-exposure-time-calculator/qualification/>)

## 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`