Overview
ISO 2720:1974 - "Photography - General purpose photographic exposure meters (photoelectric type) - Guide to product specification" defines calibration levels and test conditions for photoelectric exposure meters used to determine camera exposure. It applies to meters that measure reflected light, incident light, or both, and that contain a light‑sensitive element, an indicating electrical instrument, a directional system and a calculator to correlate meter readings with camera settings. The standard does not cover automatic or semi‑automatic camera exposure control.
Key topics and technical requirements
- Scope and markings
- Requirement for clear nomenclature, film speed (logarithmic/arithmetic, ISO designations), exposure value (EV/STEP) and aperture/shutter scales compatible with clause 6 formulae.
- Calibration and constants
- Defines calibration constants (K for reflected meters, C for incident meters) and permissible marking/labeling of these values.
- Light sources and colour temperature
- Reference lamp/filter combinations to simulate practical daylight/tungsten mix (examples: correlated colour temperature near 4 700 K; tungsten lamp at 2 855.6 K with filter yielding transmittance 0.259).
- Spectral sensitivity
- Continuous spectral response required over 400–700 nm; spectral response assessed against a colour‑conversion filter and CIE standard photometric observer.
- Performance tests
- Response time at normal and low light levels; fatigue of photo‑electric element; low‑light recovery.
- Shock resistance: one half‑sine impact at 75 g, duration 3.5 ms, applied to six faces.
- Vibration: IEC 68‑2‑6 style vibration (30→100→30 Hz cycles, applied in three orthogonal axes).
- Humidity: prolonged dark exposure at high humidity (e.g., 40 h at 35 °C, 90% RH) and subsequent calibration check.
- Temperature: checks at 0 °C to +40 °C and extremes (−35 °C and +50 °C) for indication drift.
- Directional characteristics
- Defines specific/oblique acceptance angles and average acceptance for meter directional response.
- Practical considerations
- Battery life guidance, zero adjustment, bearing friction elimination during readings.
Applications and users
- Manufacturers of photoelectric exposure meters and handheld light meters
- Test laboratories and calibration facilities verifying meter accuracy, spectral response and environmental resilience
- Camera and photographic equipment designers ensuring compatibility of meter output with film speed and exposure calculators
- Photographers and rental houses that require certified metering equipment for critical exposure work
Related standards and references
- ISO/TC 42 (Photography) - technical committee responsible for development
- IEC Publication 68‑2‑6 (Vibration) and IEC 68‑2‑27 (Shock) - referenced test procedures
- CIE standard photometric observer (for spectral assessments)
Keywords: ISO 2720:1974, photographic exposure meters, photoelectric exposure meters, calibration, test conditions, reflected light meter, incident light meter, spectral sensitivity, exposure value, film speed.