{"uid":"cap_2jPMprZxHCMfjMeDDsuqY","slug":"photography-calculators-flash-guide-number-d4735638","name":"Photography Calculators – Flash Guide Number","description":"Flash guide-number maths: range for an aperture and ISO, or the aperture needed for a distance, in metres and feet.","url":"https://photo-calc.x402tookit.com/flash-guide-number?utm_source=zero.xyz","method":"GET","headers":{},"bodySchema":{"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"]},"quarterPowerRangeM":{"type":["null","number"]}}}}}}},"responseSchema":null,"example":null,"exampleRequest":null,"tags":["x402"],"displayCostAmount":"0.02","displayCostAsset":"USDC","priceDynamic":false,"priceHint":null,"priceStatus":"priced","priceSource":"probe","requiresHandshake":false,"reviewCount":0,"rating":{"score":"0.00","successRate":"0.00","reviews":0,"stars":null,"state":"unrated"},"availabilityStatus":"unknown","priceObserved":null,"sessionDeposit":null,"pricing":{"kind":"static","summary":"$0.02/call","primary":{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.02","per":"call","confidence":"exact"},"accepted":[{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.02","per":"call","confidence":"exact"}]},"paymentMethods":[{"uid":"pm_bLNCqESInseyqdhYjVJb3","protocol":"x402","methodType":"crypto","chain":"base","mode":"charge","costAmount":"0.02","costPer":"request","priority":0,"asset":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","unit":"request","depositMicros":null,"planRef":null}],"brandName":null,"brandSlug":null,"brandBaseUrl":null,"brandDocsUrl":null,"whatItDoes":"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.","exampleAgentPrompt":"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.","exampleUseCases":[{"title":"Speedlight range check before a shoot","prompt":"I have a speedlight with guide number 58 at ISO 100 and I plan to shoot at f/4 with ISO 200 — how many metres and feet of flash range will I get?"},{"title":"Find aperture for a specific subject distance","prompt":"My flash has a guide number of 40 at ISO 100 and my subject is 3.5 metres away at ISO 100 — what aperture do I need to correctly expose them?"},{"title":"Off-camera flash planning for group portrait","prompt":"I'm setting up an off-camera flash with guide number 60 for a group 6 metres away at ISO 400 — what aperture should I use and what's the half-power range?"}],"resultDescription":"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.","failureModes":["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"],"whenToPreferThis":"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.","instructions":null,"reviewSummary":null,"reviewSummaryHighlights":null,"reviewSummaryConcerns":null,"reviewSummaryGeneratedAt":null,"activationCount":0,"lastUsedAt":null,"lastSuccessfullyRanAt":null,"lastHealthCheckAt":"2026-10-30T00:35:02.650Z","isFirstParty":false,"canonicalSlug":"photography-calculators-flash-guide-number-d4735638"}