Overview
ISO 1230:2007 - Photography - Determination of flash guide numbers for electronic flash equipment - defines the ISO guide number (GN) used in flash photography and specifies how to determine it for electronic flash units. The standard explains the physical basis of guide numbers, provides the primary calculation linking flash output to guide number, and gives rules for product designation (rounding and units). It does not cover light‑output measurement methods (these are defined in ISO 2827).
Key topics and requirements
- Definition of guide number (GN): the product of the camera lens f‑number and the distance from flash to subject for a suitable exposure at a specified ISO speed. The ISO guide number is referenced to ISO 100.
- Primary calculation: ISO 1230:2007 defines the ISO guide number by the equation
N_ISO = k × ∫ I(t) dt
where k = 0.51 (lm·m·s⁻¹) and ∫I(t)dt is the time integral of luminous intensity (expressed in cd·s). (See Annex B for the derivation.)
- Scaling for other ISO speeds: the standard provides a method to calculate guide numbers for ISO speeds other than ISO 100 by scaling the ISO‑100 value (see Section 3.2).
- Designation and presentation: guide numbers on products or instruction manuals must be rounded to two significant digits and followed by the unit metres. If quoted for a speed other than ISO 100, the reference ISO speed must be included in parentheses.
- Units conversion: Annex A gives the conversion factor from metres to feet (multiply metres by 3.28) and design rules when expressing GN in feet.
- Scope limits: ISO 1230:2007 is focused on definition and determination of GN; measurement procedures for flash light output are outside its scope (ISO 2827).
Applications and users
ISO 1230:2007 is useful for:
- Photographers and educators who need a standardized way to compare flash power across units.
- Product designers and manufacturers of electronic flash equipment for consistent labelling and specification of guide numbers.
- Technical writers, retailers, and instruction manual authors who present flash specifications.
- Test labs and standards bodies validating GN declarations.
Practical uses include exposure planning in on‑camera flash photography, specification of flash output on product datasheets, and consistent comparison of flash units from different manufacturers.
Related standards
- ISO 2827 - Electronic flash equipment: determination of light output and performance (measurement methods).
- ISO 12232, ISO 6, ISO 516 and related photographic ISO standards cited in the bibliography for exposure, ISO speed and lens/aperture definitions.
Keywords: ISO 1230:2007, flash guide number, guide number determination, electronic flash equipment, ISO guide number, photography standard, flash photography.
