Source: https://guthlabs.ai/docs/instruments/focal-length-equivalent-crop-factor-converter/mechanism/

instrument mechanism / verified-clean

# Focal Length Equivalent / Crop Factor Converter mechanism

The inspectable method record for Focal Length Equivalent / Crop Factor Converter, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The two multiplication formulas are standard under their stated assumptions. Verdict is NEEDS_REVIEW because the exact crop-factor convention and sensor-format lookup table are unspecified, and the focal-length input/output unit policy is ambiguous.

No additional formula finding is recorded.

## Units and conventions

- focal length is a linear measurement; the output focal length must use the same unit as the input unless an explicit conversion is performed

- aperture means the dimensionless f-number, such as f/2.8, not the physical entrance-pupil diameter

- crop factor is assumed to be relative to the 35mm still-image format, conventionally 36 mm × 24 mm, usually using diagonal-based comparison

- effective aperture for DoF equivalence assumes equivalent framing and comparable viewing/output conditions; it does not change exposure or the physical aperture

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "focalLength": 50,
    "focalLengthUnit": "mm",
    "apertureFNumber": 2.8,
    "sensorFormat": "APS-C",
    "cropFactor": 1.5
  },
  "expected_outputs": {
    "equivFocalLength": 75,
    "equivFocalLengthUnit": "mm",
    "equivApertureFNumber": 4.2
  },
  "working": "equivFocal = focal × cropFactor = 50 mm × 1.5 = 75 mm. equivAperture = aperture × cropFactor = 2.8 × 1.5 = 4.2, so the DoF-equivalent aperture is f/4.2."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

Extreme focal length and crop-factor inputs produce Infinity through overflow, while zero aperture produces a mathematically defined but physically invalid f/0 result. Negative focal length also exposes an unhandled sign-convention ambiguity.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/focal-length-equivalent-crop-factor-converter/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/focal-length-equivalent-crop-factor-converter/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/focal-length-equivalent-crop-factor-converter/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-tranche5/tranche5-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`