# Photography Calculators – Flash Guide Number

> Photography Calculators – Flash Guide Number is a paid API for AI agents from photo-calc.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-30).

Calculates flash effective range for a given aperture and ISO, or the aperture needed to correctly expose a subject at a given distance, using flash guide-number mathematics.

## Facts

- Endpoint: GET https://photo-calc.x402tookit.com/flash-guide-number?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-30
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/photography-calculators-flash-guide-number-d4735638
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_2jPMprZxHCMfjMeDDsuqY

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability photography-calculators-flash-guide-number-d4735638
```

Example prompt: My flash has a guide number of 58 at ISO 100 — if I'm shooting at f/5.6 and ISO 400, how far will it reach? Give me the range in both metres and feet.

## When to prefer this

Use this endpoint when you need precise flash guide-number arithmetic — finding how far a flash will reach at a chosen aperture and ISO, or finding the aperture needed for a subject at a known distance. It is purpose-built for flash exposure maths and returns both metric and imperial results with power-level range variants, making it more specific than a general exposure calculator.

## Known failure modes

- Missing required guideNumber parameter returns an error
- Providing neither aperture nor distanceM (or both simultaneously) may produce ambiguous or error results
- Negative or zero values for guide number, ISO, aperture, or distance return validation errors
- Extreme input values (very high ISO or very large distances) may return degraded or out-of-range results

## How this service works

Flash guide-number maths: range for an aperture and ISO, or the aperture needed for a distance, in metres and feet.

## Output

Returns the effective flash range in metres and feet (when aperture is given), or the required f-number (when distance is given), along with the guide number adjusted for the specified ISO, half-power and quarter-power range distances, and a human-readable summary.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [
      "guideNumber"
     ],
     "properties": {
      "iso": {
       "type": "number",
       "description": "ISO (default 100)"
      },
      "aperture": {
       "type": "number",
       "description": "f-number to find the distance (give aperture or distanceM)"
      },
      "distanceM": {
       "type": "number",
       "description": "Subject distance in metres to find the aperture"
      },
      "guideNumber": {
       "type": "number",
       "description": "Guide number at ISO 100 in metres (GN 58 means 58 m at f/1)"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "asOf": {
       "type": [
        "null",
        "string"
       ]
      },
      "rangeM": {
       "type": [
        "null",
        "number"
       ]
      },
      "source": {
       "type": [
        "null",
        "string"
       ]
      },
      "rangeFt": {
       "type": [
        "null",
        "number"
       ]
      },
      "reasons": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "summary": {
       "type": [
        "null",
        "string"
       ]
      },
      "verdict": {
       "type": [
        "null",
        "string"
       ]
      },
      "aperture": {
       "type": [
        "null",
        "number"
       ]
      },
      "degraded": {
       "type": [
        "boolean",
        "null"
       ]
      },
      "disclaimer": {
       "type": [
        "null",
        "string"
       ]
      },
      "notChecked": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "halfPowerRangeM": {
       "type": [
        "null",
        "number"
       ]
      },
      "guideNumberAtIso": {
       "type": [
        "null",
        "number"
       ]
      },
      "quarterP
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/photography-calculators-flash-guide-number-d4735638/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from photo-calc.x402tookit.com](https://www.zero.xyz/host/photo-calc.x402tookit.com/llms.txt)