Overview
ISO 6:1993 - Photography - Black-and-white pictorial still camera negative film/process systems - Determination of ISO speed specifies a sensitometric method for determining and expressing the ISO speed of black-and-white negative film together with its processing system. The standard defines how to expose, process and measure negatives to derive a reproducible film speed (ISO) that manufacturers, test labs and photographers can use for labeling, quality control and exposure guidance.
Key topics and technical requirements
- Scope: Applies to pictorial still camera black-and-white negative film/process systems (excludes cinema, aerial, radiography, micrography and some transfer systems).
- Sensitometric method: Expose film with a non‑intermittent sensitometer under spectral conditions per ISO 7589, produce a sensitometric curve and measure densities to derive the speed point.
- Reference points (m and n):
- Point m: density = base+fog + 0.10
- Point n: located 1.30 log10 exposure units higher than m
- Development is selected so the density difference AD between m and n equals 0.80
- Hm = exposure (lux·s) corresponding to point m; ISO speed is calculated from log10 Hm via the standard’s conversion table
- Densitometry: Use a densitometer conforming to ISO 5-2 (geometric) and ISO 5-3 (spectral) conditions; measurements taken on uniform image areas.
- Sample conditioning and storage: Samples must represent average user lots and be stored/conditioned (23 °C ±2 °C, 50 % ±5 % RH). Ordinary films processed 5–10 days after exposure; professional films 4 hours–7 days.
- Processing documentation: Manufacturers must specify processing conditions (chemicals, times, temperature, agitation) and the illuminant used to compute ISO speed. The standard removed the earlier requirement to prescribe a single developer/fixer.
- Accuracy: Calibration of equipment and processes must ensure an error in log10 Hm
