Overview
ISO 517:2008 - Photography - Apertures and related properties pertaining to photographic lenses - Designations and measurements defines how apertures (f-numbers) and related properties of photographic lenses are designated and measured. The standard applies to lenses for still cameras focused at infinity (objects at least 50× the focal length). It sets the accepted aperture markings, tolerances for stop numbers, definitions (aperture stop, entrance/exit pupil, focal length, relative aperture, true f-number) and gives practical measurement methods.
Key topics and requirements
- Aperture designation formats: Use 1: followed by the f-number (e.g. 1:2,8) or the symbol f/ (e.g. f/2.8). Decimal points or commas are permitted.
- Standard f-number series: 0.5 – 0.7 – 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 – 32 – 45 – 64 – 90 – 128. Whole-stop values are derived by f-number = 2^(m/2) (m integer).
- Subdivisions: Half- and third-stop subdivisions are defined (geometric series rules) for 1/2 and 1/3 stop increments.
- Tolerances of measured f-numbers (to true f-number):
- Full aperture: ±5%
- Smaller than f/5.6: +12% / −11% (≈ ±1/3 stop)
- f/5.6 and larger: +19% / −16% (≈ ±1/2 stop)
- Definitions: Clear definitions of aperture stop, entrance pupil, exit pupil, focal length (limit of image size/angle) and relative aperture (twice the numerical aperture) are provided for consistent terminology.
- Measurement methods:
- Entrance pupil: direct measurement with a travelling microscope or telecentric projection system (image area, magnification calibration).
- Focal length: telescope pivot method (reticle and scale) and collimator+microscope method (using known collimator focal length and target size). Measurement guidance includes light sources (green mercury line, λ = 546 nm) and approximate lens ranges (20–500 mm focal length, apertures 5–100 mm).
Applications and who uses it
- Lens manufacturers: marking, design validation and production QA of photographic lenses.
- Optical test laboratories: verifying f-numbers, entrance pupil diameter and focal length to ISO tolerances.
- Camera and accessory designers: ensuring consistent aperture designation and optical interoperability.
- Standards bodies and certification schemes: harmonizing test methods and labeling requirements for still-camera lenses.
- Advanced photographers and educators: reference for understanding nominal vs. true f-numbers and how aperture affects exposure/illuminance.
Related standards
ISO 517:2008 was prepared by ISO/TC 172 (Optics and photonics), Subcommittee SC 1 (Fundamental standards). It complements other ISO optics and photography standards used for lens metrology and optical specifications.
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