ISO 6328:2000

INTERNATIONAL ISO
STANDARD 6328
Second edition
2000-05-01
Corrected and reprinted
2000-09-01
Photography — Photographic materials —
Determination of ISO resolving power
Photographie — Surfaces sensibles — Détermination du pouvoir résolvant
ISO
Reference number
©
ISO 2000
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ii © ISO 2000 – All rights reserved

Contents Page
Foreword.iv
Introduction.v
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Sampling and storage .3
5 Test method.3
6 Product classification .13
7 Product marking and labelling .14
Annex A (informative) Illustration of an example of a resolving-power camera .15
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 6328 was prepared by Technical Committee ISO/TC 42, Photography.
This second edition cancels and replaces the first edition (ISO 6328:1982), which has been technically revised.
In this revision, the description of the process of evaluating images has been expanded. The specifications of the
resolving power camera’s objectives have been altered slightly, and the illustration of a resolving-power camera
has been moved to informative annex A. The text describing qualification of the camera has been expanded to
include the target.
This second edition also contains many other changes, most of which were made for the sake of clarity of
understanding. Among these changes is a clarification of part of the scope. The definitions have also been
expanded; some terms have been substituted for others in order for their usage to be more consistent and their
meaning to be clearer. The term “element” has been introduced in place of “test pattern” and “tribar resolving-
power target” replaces “test chart”. Other terms introduced or clearly defined include “target polarity”, “limiting
element” and “ISO resolving power”.
Annex A of this International Standard is for information only.
iv © ISO 2000 – All rights reserved

Introduction
The resolving power of a photographic material is used to estimate the smallest detail that may be visually
observable when recorded on the material. It combines the effects of modulation transfer function, graininess and
contrast, all of which contribute to overall image quality, and human observers, each of whom may differ in their
assessment of quality. The method is particularly useful for appraising materials that will be viewed at high
magnification such as microfilm, 8 mm and 16 mm motion picture film, etc. However, resolving power should not
be expected to predict overall image quality in every situation, because image quality is too complex to be
described by a single factor. This is particularly the case for low-contrast continuous-tone products.
Resolving power as measured by photographing suitable tribar resolving-power targets is very dependent on
conditions of measurement, and the structure of the target element. It depends markedly on the photographic
conditions employed and on the presence of background glare from the illuminated target. It is affected by such
factors as the spectral content of the light used, the exposure level, the focus, processing procedures, the lens
aperture at which the test is made, the contrast of the target and the magnification of the camera lens and the
microscope through which the images are observed, etc.
The judgment exercised by the human observer in determining resolving power can be a source of significant
experimental error. The criterion of resolution given in this International Standard was selected because it
appeared to admit less latitude in interpretation than others. The description of the process of evaluating images
has been expanded in this revision.
INTERNATIONAL STANDARD ISO 6328:2000(E)
Photography — Photographic materials — Determination of ISO
resolving power
1 Scope
This International Standard specifies a method for determining the resolving power of photographic films, plates
and papers, including black-and-white films, black-and-white printing papers, colour-reversal films, colour-negative
films, and colour-printing papers.
Materials designed for X-ray and other high-energy radiation are excluded, as are photographic materials used in
medical radiography where the exposure source is an intensifying screen in contact with the film (sensitized on one
or two sides). Also excluded are materials having photo-polymer, diazo, etc. light-sensitive layers.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative documents indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 497:1973, Guide to the choice of series of preferred numbers and of series containing more rounded values of
preferred numbers.
ISO 554:1976, Standard atmospheres for conditioning and/or testing — Specifications.
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply.
3.1
element
three parallel bars and two spaces of equal width and separation
3.2
group number
number of square dots preceding the array of elements which is used to locate a position on the tribar resolving-
power target
3.3
tribar resolving-power target
array of identical elements that decrease in size (geometrically), and typically spirals towards the centre of the
target format
3.4
spatial period
within an element of a tribar target, the distance between the leading edges of two consecutive bars
NOTE The spatial period is usually expressed in millimetres (mm).
3.5
spatial frequency
reciprocal of the spatial period denoting the number of identical bar and space pairs that can be contained within an
overall width of 1 mm
NOTE The spatial frequency is expressed as reciprocal millimetres (cycles per millimetre).
3.6
contrast ratio
ratio of the luminances of the bars of the element to the luminance of the surround or the antilog of the density
difference between the bar and its surround
3.7
target polarity
transmission relationship of the parallel bars and their surround
3.7.1
negative polarity
light bars against a dark surround
3.7.2
positive polarity
dark bars against a light surround
3.8
camera
optical system by which the tribar resolving-power target is imaged and recorded, with suitable reduction in size, on
the photographic material being tested
3.9
reference surface
flat surface against which the emulsion side of the photographic material is pressed during exposure
3.10
qualification
�camera�
attainment of the necessary high optical performance of a camera, essential for its use in determining resolving
power
3.11
replicate set
series of images of the tribar resolving-power target made at the same focus and exposure settings
3.12
exposure series
series of images made at different exposure settings
3.13
focus series
series of images made at different focus settings
3.14
resolving power
ability of a photographic material to maintain, in the developed image, the separate identity of parallel bars when
their separation is small
NOTE The resolving power is numerically equal to the spatial frequency of the smallest element that can be resolved.
2 © ISO 2000 – All rights reserved

3.15
limiting element
that element in the image of the tribar resolving-power target selected by the observer, with reasonable confidence,
to be at the threshold of no longer being able to distinguish three bars and two spaces, even if the number were not
known to be three
3.16
resolving power of a replicate set
median of the resolving powers of the test material in images of the replicate set
3.17
maximum resolving power
resolving power of the test material under conditions of optimum focus and exposure and the test conditions
defined in this International Standard
3.18
ISO resolving power
averaged maximum resolving power, rounded according to Table 5 and ISO 497
NOTE There is an ISO high-contrast and an ISO low-contrast resolving power.
4 Sampling and storage
In determining the resolving power of a product, it is important that the samples evaluated yield the average results
obtained by users. This will require evaluating several different batches periodically under the conditions specified
in this International Standard.
Prior to evaluation, the samples shall be stored according to the manufacturer’s recommendations for a length of
time that simulates the average age at which the product is normally used. When no specific recommendation is
made, storage shall comply with the specifications of ISO 554, 23 °C � 2 °C and a relative humidity of (50 � 5) %.
Several independent evaluations shall be made to ensure the proper calibration of equipment and processes. The
basic objective in selecting and storing samples as described above is to ensure that the material characteristics
are representative of those obtained by a consumer at the time of use.
5 Test method
5.1 Principle
The resolving power of a material is determined by visual inspection of the image of the tribar resolving-power
target recorded on the test material by means of a suitable camera system. It depends on the contrast ratio,
polarity and exposure. The resolving power passes through a maximum as the exposure is changed from a value
at the toe of the characteristic curve to a value toward the shoulder. Furthermore, the resolving power passes
through a maximum as the focus setting is given successive values that vary from one side of the correct focus to
the other.
In brief, the procedure is to first determine the exposure that gives the best-resolved image of the tribar resolving-
power target, using the focus setting determined during the camera qualification test. A through focus series is
then run at this exposure level to optimize focus. The maximum resolving power obtained from this series is used
to determine the ISO resolving power of a sample using the scale defined in Table 5.
Because of the granular structure of the image when viewed under magnification, a replicate set often yields a
range of resolving power values. To mitigate the effect of this variable, ISO resolving power is defined below in
terms of the median value of a set of not le
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