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← Standards

ISO 19264-1

ISO 19264-1:2021 — Photography — Archiving systems — Imaging systems quality analysis — Part 1: Reflective originals — is the international standard for measuring the image quality of the equipment used to digitise cultural heritage. It was developed by ISO/TC 42, the committee for photography, in a joint working group (JWG 26) of experts nominated by national standards bodies, and published in June 2021.

It is the common ground under the other two standards on these pages. FADGI states that its guidelines are aligned with it, and Metamorfoze 2.0 brought its measuring methods into line with it.

PHOTARCH does not assess objects by ISO 19264-1. It assesses them by its own three levels — record shot, study and reproduction, set out under PHOTARCH's levels — and offers ISO 19264-1, with FADGI and Metamorfoze, for paintings, documents, prints and other flat originals: a setting in the PHOTARCH app turns them on, clearly labelled. This page explains the standard itself.

What it measures — and what it does not

ISO 19264-1 specifies a method for analysing the quality of an imaging system — a camera or scanner with its lens, light and software — from a single image of a specified test target. It defines which characteristics are measured, how they are measured, and how the results are reported.

That is its strength and its limit. It answers the question is this set-up capable of a faithful reproduction? It does not look at the photograph of a particular original, and it says nothing about how that original should be placed, lit or framed. Part 1 covers reflective originals — paper, paintings, prints, the surfaces of objects. Transmissive originals such as film are to be covered by a further part.

The characteristics

The analysis covers, among others, the properties that together decide whether a reproduction is faithful:

Characteristic What it tells you
Tone reproduction (OECF) Whether greys come out at their true lightness
Gain modulation Whether tonal differences survive, especially in the highlights
Dynamic range How deep a black is still recorded with detail
White balance Whether neutrals stay neutral
Illumination non-uniformity Whether the light falls evenly across the field
Colour reproduction How far colours land from their reference, as ΔE
Sampling rate and resolution (SFR / MTF) How much fine detail is recorded, and how sharply
Sharpening Whether edges have been artificially boosted
Noise Random variation in flat areas
Colour misregistration Whether the three colour channels line up
Geometric distortion Whether straight lines stay straight and scale stays even

Levels A, B and C

The standard groups its tolerances into three levels, A (the strictest), B and C. The tolerances are given in an informative annex (Annex B): they are recommendations an institution can adopt or tighten, not limits the standard imposes.

FADGI maps its star levels onto them: four-star corresponds to Level A, three-star to Level B and two-star to Level C. FADGI's one-star level has no ISO equivalent.

For orientation, a few of the Level A tolerances, as cited in the Metamorfoze guidelines (§2.27):

Characteristic Level A
Colour reproduction mean ΔE ≤ 4, maximum ΔE ≤ 10
Colour misregistration ≤ 0.4 pixel
Geometric distortion ≤ 1.5 %
Gain modulation, L* 85 to the darkest patch 70–130 %
Dynamic range density 2.3

Getting hold of it

ISO standards are sold, not published freely. ISO 19264-1 can be bought from ISO or from a national standards body, such as SIS in Sweden or BSI in the United Kingdom. The free FADGI and Metamorfoze guidelines explain most of what it means in practice.

How to relate to it

  • It qualifies the set-up, not the picture. Test the camera, lens and light with a target regularly, and again whenever anything changes. A good result means the set-up can reproduce faithfully; it is the work in front of it that decides whether a given photograph does.
  • Choose a level as a policy. Level B for most collection work, Level A where colour and detail are the subject — the same choice as FADGI's three- and four-star.
  • Record the evidence. Which target, which software, which date, which result. That record is what turns a claim of ISO conformance into something another institution can check.

What PHOTARCH measures, and what it does not

The PHOTARCH app does not analyse a test target under ISO 19264-1. With the setting for flat originals on, it reports an ISO level through FADGI's correspondence: the FADGI §3.13 grade it measures — format, colour space, the bit depth of the file with the bits the camera recorded beside it, resolution at the object, and colour on the sitting's ColorChecker — is translated to Level A, B or C. Everything FADGI's grade leaves unmeasured is unmeasured here too: sharpness on a target, sharpening, noise, uniformity, misregistration, scale accuracy and gain modulation. Flat originals are lit evenly from two sides; how evenly is not measured.

An ISO level in PHOTARCH is therefore a derived indication — useful for seeing where a photograph stands, and labelled as such — never a measurement made under the standard.

Sources

← FADGIMetamorfoze →