ISO 6328:2000, "Photography - Photographic materials - Determination of ISO resolving power," defines a standardized method to measure the resolving power of photographic films, plates and papers (black‑and‑white and colour reversal/negative/printing papers). The standard specifies the use of tribar resolving‑power targets, a qualified imaging camera, controlled exposures and microscopic visual inspection to identify the finest element that can be resolved on the developed material. Materials for X‑ray/high‑energy radiation, medical radiography with intensifying screens, and photo‑polymer/diazo‑type layers are excluded.
Key topics and technical requirements
- Test principle: Image a tribar target onto the photographic material, determine the finest element resolved after processing by microscopic/visual readout, and derive spatial frequency (cycles/mm).
- Tribar element geometry: Each element is three parallel bars and two spaces with a bar length:width aspect ratio of 5:1; element square dimension equals 2.5 × period (L).
- Spatial frequency measurement: Calculate as the reciprocal of the period (1/L) or from overall element width via spatial frequency = 2.5 / (overall element width).
- Target design: Array of geometrically decreasing elements (array ≈ 100 mm square, surround ≈ 125 mm). Successive elements increase spatial frequency by about 12%.
- Target properties: Spectrally neutral transparency; available in positive and negative polarity and in defined contrast ratios.
- Camera qualification & test protocol: Determine optimal exposure and focus by running exposure and focus series; use a qualified resolving‑power camera and reference surface during exposure.
- Replicate principle & statistics: Use replicate sets (not less than nine images at same focus/exposure). ISO resolving power is the averaged maximum resolving power (median of replicate set) rounded per ISO 497.
- Environmental control and sampling: Samples stored per manufacturer guidance or ISO 554 (recommended 23 °C ± 2 °C and 50% ± 5% RH) and multiple batches should be evaluated to represent user experience.
- Observer variability: The method provides guidelines to reduce subjective error from visual judgment.
Applications and practical value
- Quality assurance and production control for photographic films, plates and papers.
- Performance specification for microfilm, motion‑picture film (8/16 mm) and other high‑magnification imaging media.
- Test procedures for R&D, acceptance testing, and batch certification in photographic manufacture and imaging labs.
- Useful for archivists, imaging test laboratories, forensic imaging, and conservation professionals who need objective resolving‑power data.
Who should use this standard
- Photographic material manufacturers and QC labs
- Independent testing and calibration laboratories
- Film processing and motion‑picture labs
- Archivists and records‑management professionals
- Standards bodies and technical committees working on photographic imaging
Related standards
- ISO 554 - Standard atmospheres for conditioning and/or testing (storage conditions referenced)
- ISO 497 - Rules for rounding and preferred numbers (used for reporting ISO resolving power)
Keywords: ISO 6328, resolving power, photographic materials, tribar resolving‑power target, spatial frequency, resolving‑power camera, photographic film testing, ISO resolving power.
