Overview
ISO 11421:1997 - "Optics and optical instruments - Accuracy of optical transfer function (OTF) measurement" gives practical guidance for evaluating and expressing the accuracy and uncertainty of OTF measurements, with emphasis on the modulation transfer function (MTF) component. The standard identifies main sources of measurement error in OTF measuring equipment, provides methods to assess those errors, and explains how to combine them to estimate overall measurement uncertainty. The phase transfer function (PTF) is treated in Annex A.
Key topics and technical requirements
- Scope and focus
- Concentrates on MTF measurement accuracy; PTF guidance is in Annex A.
- Encourages realistic uncertainty statements that reflect both equipment and test-piece influences.
- Definitions and symbols
- Defines terms such as standard lens (MTF predictable to better than 0.05 MTF units) and audit lens, and lists symbols for parameters used in error calculations.
- Sources of inaccuracy
- Detailed classification of error sources in OTF measuring equipment: optical bench geometry (slide straightness, perpendicularity), focus and conjugate errors (finite/infinite object conjugates), azimuth/rotation errors, mounting/tilt of test piece, orientation of test target and analyser (slit/grating alignment), and special cases (image intensifiers).
- Error assessment and calculation
- Provides methods and equations to quantify how individual errors (focus change, angular misalignment, spatial frequency error) affect MTF.
- Describes combining systematic and random error components to calculate overall accuracy and uncertainty.
- Specification and routine evaluation
- Guidance on specifying a general equipment accuracy value and recommended procedures for routine performance checks.
- Annexes
- Annex A: PTF accuracy
- Annex B: rate-of-change determinations
- Annex C: example calculations
- Annex D: bibliography
Applications and users
ISO 11421 is directly relevant to:
- Optical and electro‑optical test laboratories performing MTF/OTF measurements
- Manufacturers of lenses, imaging sensors, cameras, and test equipment who need to specify, verify and calibrate measurement systems
- National standards and calibration laboratories (audit and standard lens characterization)
- R&D and QA teams assessing image-forming performance and reporting measurement uncertainty
Practical uses include benchmarking imaging system resolution, validating test benches, preparing uncertainty budgets, and routine equipment performance evaluation.
Related standards
- ISO 9334 - definitions and mathematical relationships for OTF
- ISO 9335 - principles and procedures for OTF measurement
- ISO 9336 - application-specific OTF measurement guidance (multiple parts)
Keywords: ISO 11421, optical transfer function, OTF, MTF measurement accuracy, measurement uncertainty, optics standard, optical instruments, bench geometry, slit alignment.