ISO 2721:2013

Overview

ISO 2721:2013 - Photography - Film-based cameras - Automatic controls of exposure specifies nominal exposure values and test methods for automatic exposure control systems in film-based cameras. It defines how to measure and calibrate exposure at the focal plane as a function of field luminance and film speed, describes photometric characteristics such as acceptance angles of the photoelectric system, and sets requirements for sensor spectral response, out‑of‑range indicators and calibration conditions. This standard applies only to film-based cameras (not digital cameras).

Key topics and technical requirements

• Nominal focal‑plane exposure: The standard gives a reference nominal exposure for a defined luminance range (4 to 4 096 cd/m²) and film speeds; a reference constant H0 = 10 lx·s is used to set nominal values. The nominal value is intended as a design and test reference rather than an absolute “correct” exposure.
• Tolerance and special cases: A typical calibration tolerance is ±1 exposure value (±1 EV); tighter tolerances (e.g., ±1/3 EV) may be required for some film types. Cine cameras (8 mm / Super 8) may use an increased nominal exposure (≈1.26×, i.e., +1/3 EV).
• Calibration method: Exposure must be measured in the focal plane using a circular measuring area concentric with the lens axis (diameter = three‑quarters of the shorter side of the nominal picture format, with guidance for medium/large format cameras). Calibration uses an instrument with a photopic (CIE V(λ)) or correlatable spectral response.
• Calibration light source: A Lambertian source with continuous visible spectrum, uniform luminance ±4%, chromaticity close to 4 700 K (±200 K) and controlled spectral radiance is required. Source field must exceed the camera’s photometric field.
• Sensor spectral sensitivity: Response shall be continuous across 380–780 nm. For daylight/tungsten compatibility the ratio of sensor responses at standard color temperatures must be within ±1/3 EV; less than 10% of response should come from wavelengths >700 nm or <380 nm under test conditions.
• User indicators: For cameras using colour reversal film, an out‑of‑range indicator is recommended when scene luminance exceeds exposure capability by >1 EV; a long‑exposure signal is recommended for exposures longer than about 1/30 s.
• Additional test provisions: Acceptance angles of the photoelectric system, oblique acceptance angles, and performance tests are specified; normative annexes cover focal‑plane measurement, measuring area size and performance testing.

Practical applications

• Design and validation of automatic exposure systems for film cameras
• Factory calibration and quality control for camera production
• Service, repair and recalibration of vintage and current film cameras
• Test-lab procedures for conformity testing and performance benchmarking
• Guidance for sensor selection, optical system design and viewfinder indicators

Who should use this standard

• Camera manufacturers and optical/electronics designers
• Calibration and test laboratories
• Service/repair technicians for film cameras
• Standards and compliance professionals in photographic equipment
• Archivists and restorers working with film imaging equipment

Related standards

• ISO 2720 (scales for f‑number, exposure time, exposure value, film speed)
• IEC 60068‑2‑6 and IEC 60068‑2‑27 (environmental vibration and shock tests)
• Annexes in ISO 2721:2013 (A–C) provide measurement, measuring-area and performance test details

Keywords: ISO 2721:2013, automatic exposure control, film-based cameras, focal plane exposure, calibration, spectral sensitivity, acceptance angle, photometric testing.