Flash Exposure Calculator
Nominal direct flash fnumberGN/d×√(ISO/ISOref), actual guide-number units/distance/reference independently established. Bounce/modifiers/nonuniform exposure external.
Description
A direct-flash guide-number estimate relates aperture, subject distance and the chosen ISO reference.
A direct-flash guide-number estimate relates aperture, subject distance and the chosen ISO reference.1
This tool evaluates that supplied relation without accounting for bounce paths, modifiers or exposure uniformity.
When to use Flash Exposure Calculator
Use this tool to evaluate that supplied relation without accounting for bounce paths, modifiers or exposure uniformity.
How to use Flash Exposure Calculator and how it works
- Prepare callerMeasuredGuideNumberAtReferenceIso, actualIso, callerGuideNumberReferenceIso, actualFlashSubjectDistance, with the units, conventions and assumptions below.
- Enter the required fields and run Flash Exposure Calculator.
- Read result.value using the output definitions below.
- Correct any rejected inputs and rerun; interpret the result within the stated model and its assumptions.
| Input field | Meaning | Value type | Requirement |
|---|---|---|---|
| callerMeasuredGuideNumberAtReferenceIso | Caller Measured Guide Number At Reference Iso. Use the units and matching context stated in the calculation below. | number | Required; strictly positive; finite; other constraints below |
| actualIso | Actual Iso. Use the units and matching context stated in the calculation below. | number | Required; strictly positive; finite; other constraints below |
| callerGuideNumberReferenceIso | Caller Guide Number Reference Iso. Use the units and matching context stated in the calculation below. | number | Required; strictly positive; finite; other constraints below |
| actualFlashSubjectDistance | Actual Flash Subject Distance. Use the units and matching context stated in the calculation below. | number | Required; strictly positive; finite; other constraints below |
Supply positive guide number, target ISO, guide-number reference ISO and subject distance, using the guide number's matching distance unit. Return guideNumber/distance*sqrt(targetISO/referenceISO) as an f-number. Reflectors, bounce, diffusion and actual light output are outside this direct-flash guide-number model.
fNumber=guideNumber/distance*sqrt(targetISO/referenceISO) Apply the input ordering, units and branch conditions stated above.
| Output field | Meaning | Value type |
|---|---|---|
| result | Object containing the calculated or selected fields below. | object |
| result.value | Conditional dimensionless f-number. | number |
{
"input": {
"callerMeasuredGuideNumberAtReferenceIso": 20,
"actualIso": 100,
"callerGuideNumberReferenceIso": 100,
"actualFlashSubjectDistance": 2
},
"output": {
"result": {
"value": 10
}
}
}Limitations and assumptions
The calculation uses the supplied observations and model inputs. It does not collect measurements, select a reference population or coefficient, or establish that the model applies to the actual case. The stated branch conditions and output units remain part of the result.
All fields require explicit values. Unexpected fields, wrong types, nonfinite numbers and sparse arrays are rejected. Alignment, bounds and supported-token rules are explained above; extremely large inputs or nonfinite results can also fail. Input errors differ from successful false comparisons, unavailable-domain flags and empty match lists.
Alternative or Complementary methods
Use Flash Sync Speed Calculator for a complementary operation: A nominal full-open mechanical flash-sync frequency depends on curtain travel and an independently required margin.
Use Depth of Field Far Limit Calculator for a complementary operation: A thin-lens far depth-of-field limit depends on a separately established hyperfocal model and matched distance units.
References
-
Flash Exposure Calculator — tool.io
Similar or alternative tools
- Flash Sync Speed Calculator
A nominal full-open mechanical flash-sync frequency depends on curtain travel and an independently required margin.
- Depth of Field Far Limit Calculator
A thin-lens far depth-of-field limit depends on a separately established hyperfocal model and matched distance units.