SIST ISO 12233:2014

INTERNATIONAL ISO
STANDARD 12233
Second edition
2014-02-15
Photography — Electronic still picture
imaging — Resolution and spatial
frequency responses
Photographie — Imagerie des prises de vues électroniques —
Résolution et réponses en fréquence spatiale
Reference number
ISO 12233:2014(E)
©
ISO 2014

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ISO 12233:2014(E)

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© ISO 2014
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ISO 12233:2014(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test conditions . 5
4.1 Test chart illumination . 5
4.2 Camera framing and lens focal length setting . 5
4.3 Camera focusing . 5
4.4 Camera settings . 5
4.5 White balance. 6
4.6 Luminance and colour measurements . 6
4.7 Gamma correction . 6
5 Visual resolution measurement . 6
5.1 General . 6
5.2 Test chart. 7
5.3 Rules of judgement for visual observation . 8
6 Edge-based spatial frequency response (e-SFR) . 9
6.1 General . 9
6.2 Methodology . 9
7 Sine-based spatial frequency response (s-SFR) measurement .12
8 Presentation of results .13
8.1 General .13
8.2 Resolution .13
8.3 Spatial frequency response (SFR) .14
Annex A (informative) CIPA resolution test chart .17
Annex B (informative) Visual resolution measurement software .23
Annex C (informative) Low contrast edge SFR test chart with OECF patches .28
Annex D (normative) Edge spatial frequency response (e-SFR) algorithm .30
Annex E (normative) Sine wave star test chart .33
Annex F (normative) Sine wave Spatial Frequency Response (s-SFR) analysis algorithm .35
Annex G (informative) Colour-filtered resolution measurements .40
Annex H (informative) Units and summary metrics .42
Annex I (informative) Original test chart defined in ISO 12233:2000 .45
Bibliography .49
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ISO 12233:2014(E)

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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 42, Photography.
This second edition cancels and replaces the first edition (ISO 12233:2000), which has been technically
revised.
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ISO 12233:2014(E)

Introduction
0.1 Purpose
The spatial resolution capability is an important attribute of an electronic still-picture camera. Resolution
measurement standards allow users to compare and verify spatial resolution measurements. This
International Standard defines terminology, test charts, and test methods for performing resolution
measurements for analog and digital electronic still-picture cameras.
0.2 Technical background
For consumer digital cameras, the term resolution is often incorrectly interpreted as the number of
addressable photoelements. While there are existing protocols for determining camera pixel counts,
these should not be confused with the interpretation of resolution as addressed in this International
Standard. Qualitatively, resolution is the ability of a camera to optically capture finely spaced detail, and
is usually reported as a single valued metric. Spatial frequency response (SFR) is a multi-valued metric
that measures contrast loss as a function of spatial frequency. Generally, contrast decreases as a function
of spatial frequency to a level where detail is no longer visually resolved. This limiting frequency value
is the resolution of the camera. A camera’s resolution and its SFR are determined by a number of factors.
These include, but are not limited to, the performance of the camera lens, the number of addressable
photoelements in the optical imaging device, and the electrical circuits in the camera, which can include
image compression and gamma correction functions.
While resolution and SFR are related metrics, their difference lies in their comprehensiveness and
utility. As articulated in this International Standard, resolution is a single frequency parameter that
indicates whether the output signal contains a minimum threshold of detail information for visual
detection. In other words, resolution is the highest spatial frequency that a candidate camera can
usefully capture under cited conditions. It can be very valuable for rapid manufacturing testing, quality
control monitoring, or for providing a simple metric that can be easily understood by end users. The
algorithm used to determine resolution has been tested with visual experiments using human observers
and correlates well with their estimation of high frequency detail loss.
SFR is a numerical description of how contrast is changed by a camera as a function of the spatial
frequencies that describe the contrast. It is very beneficial for engineering, diagnostic, and image
evaluation purposes and serves as an umbrella function from which such metrics as sharpness and
acutance are derived. Often, practitioners will select the spatial frequency associated with a specified
SFR level as a modified non-visual resolution value.
In a departure from the first edition of this International Standard, two SFR measurements are described.
Additionally, the first SFR metrology method, edge-based spatial frequency response, is identical to
that described in the first edition, except that a lower contrast edge is used for the test chart. Regions
of interest (ROI) near slanted vertical and horizontal edges are digitized and used to compute the SFR
levels. The use of a slanted edge allows the edge gradient to be measured at many phases relative to the
image sensor photoelements and to yield a phase-averaged SFR response.
A second sine wave-based SFR metrology technique is introduced in this edition. Using a sine wave
modulated target in a polar format (e.g. Siemens star), it is intended to provide an SFR response that
is more resilient to ill-behaved spatial frequency signatures introduced by the image content driven
processing of consumer digital cameras. In this sense, it is intended to enable easier interpretation of
SFR levels from such camera sources. Comparing the results of the edge-based SFR and the sine based
SFR might indicate the extent to which nonlinear processing is used.
The first step in determining visual resolution or SFR is to capture an image of a suitable test chart with
the camera under test. The test chart should include features of sufficiently fine detail and frequency
content such as edges, lines, square waves, or sine wave patterns. The test chart defined in this
International Standard has been designed specifically to evaluate electronic still-picture cameras. It has
not necessarily been designed to evaluate other electronic imaging equipment such as input scanners,
CRT displays, hard-copy printers, or electro-photographic copiers, nor individual components of an
electronic still-picture camera, such as the lens.
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ISO 12233:2014(E)

Some of the measurements described in this International Standard are performed using digital analysis
techniques. They are also applicable with the analogue outputs of the camera by digitizing the analogue
signals if there is adequate digitizing equipment.
0.3 Methods for measuring SFR and resolution — selection rationale and guidance
This section is intended to provide more detailed rationale and guidance for the selection of the different
resolution metrology methods presented in this International Standard. While resolution metrology of
analog optical systems, by way of spatial frequency response, is well established and largely consistent
between methodologies (e.g. sine waves, lines, edges), metrology data for such systems is normally
captured under well controlled conditions where the required data linearity and spatial isotropy
assumptions hold. Generally, it is not safe to assume these conditions for files from many digital
cameras, even under laboratory capture environments. Exposure and image content dependent image
processing of the digital image file before it is provided as a finished file to the user prevents this. This
processing yields different SFR responses depending on the features in the scene or in the case of this
International Standard, the target. For instance, in-camera edge detection algorithms might specifically
operate on edge features and selectively enhance or blur them based on complex nonlinear decision
rules. Depending on the intent, these algorithms might also be tuned differently for repetitive scene
features such as those found in sine waves or bar pattern targets. Even for constrained camera settings
recommended in this International Standard, these nonlinear operators can yield differing SFR results
depending on the target feature set. Naturally, this causes confusion on which targets to use, either
alone or in combination. Guidelines for selection are offered below.
Edges are common features in naturally occurring scenes. They also tend to act as visual acuity cues
by which image quality is judged and imaging artefacts are manifested. This logic prescribed their use
for SFR metrology in the past and current editions of this International Standard. It is also why edge
features are prone to image processing in many consumer digital cameras: they are visually important.
All other imaging conditions being equal, camera SFRs using different target contrast edge features can
be significantly different, especially with respect to their morphology. This is largely due to nonlinear
image processing operators and would not occur for strictly linear imaging systems. To moderate this
behaviour, a lower contrast slanted edge feature (Figure C.1) was chosen to replace the higher contrast
version of the first edition. This feature choice still allows for acuity-amenable SFR results beyond the
half-sampling frequency and helps prevent nonlinear data clipping that can occur with high contrast
target features. It is also a more reliable rendering of visually important contrast levels in naturally
occurring scenes.
Sine wave features have long been the choice for directly calculating SFR of analogue imaging systems
and they are intuitively satisfying. They have been introduced into this edition of this International
Standard based on experiences from the edge-based approach. Because sine waves transition more
slowly than edges, they are not prone to being identified as edges in embedded camera processors.
As such, the ambiguity that image processing imposes on the SFR can be largely avoided by their use.
Alternatively, if the image processing is influenced by the absence of sharp features, more aggressive
processing might be used by the camera. A sine wave starburst test pattern (Figure 6) is adopted in
this edition. With the appropriate analysis software, a sine wave-based SFR can be calculated up to the
half-sampling frequency. For the same reasons stated above, the sine wave-based target is also of low
contrast and consistent with that of the edge-based version. An added benefit of the target’s design over
other sine targets is its compactness and bi-directional features.
All experience suggests that there is no single SFR for today’s digital cameras. Even under the strict
capture constraints suggested in this International Standard, the allowable feature sets that most
digital cameras offer prevent such unique characterization. Confusion can be reduced through complete
documentation of the capture conditions and camera setting for which the SFR was calculated. It has been
suggested that comparing edge- and sine wave-based SFR results under the same capture conditions
could be a good tool in assessing the contribution of spatial image processing in digital cameras.
Finally, at times, a full SFR characterization is simply not required, such as in end of line camera assembly
testing. Alternately, SFR might be an intimidating obstacle to those not trained in its utility. For those
in need of a simple and intuitive space domain approach to resolution using repeating line patterns, a
visual resolution metric is also provided in this second edition of this International Standard.
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With such a variety of methods available for measuring resolution, there are bound to be differences
in measured resolution results. To benchmark the likely variations, the committee has published the
results of a pilot study using all of the proposed techniques and how they relate to each other. These
results are provided in Reference [20].
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INTERNATIONAL STANDARD ISO 12233:2014(E)
Photography — Electronic still picture imaging —
Resolution and spatial frequency responses
1 Scope
This International Standard specifies methods for measuring the resolution and the SFR of electronic
still-picture cameras. It is applicable to the measurement of both monochrome and colour cameras
which output digital data or analog video signals.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 14524, Photography — Electronic still-picture cameras — Methods for measuring opto-electronic
conversion functions (OECFs)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
addressable photoelements
number of active photoelements in an image sensor
Note 1 to entry: This equals the product of the number of active photoelement lines and the number of active
photoelements per line.
3.2
aliasing
output image artefacts that occur in a sampled imaging system due to insufficient sampling
Note 1 to entry: These artefacts usually manifest themselves as moiré patterns in repetitive image features or as
jagged stair-stepping at edge transitions.
3.3
cycles per millimetre
cy/mm
spatial frequency unit defined as the number of spatial periods per millimetre
3.4
edge spread function
ESF
normalized spatial signal distribution in the linearized output of an imaging system resulting from
imaging a theoretical infinitely sharp edge
3.5
effectively spectrally neutral
having spectral characteristics which result in a specific imaging system producing the same output as
for a spectrally neutral object
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3.6
electronic still-picture camera
camera incorporating an image sensor that outputs an analog or digital signal representing a still picture
Note 1 to entry: This camera may also record or store an analog or digital signal representing a still picture on a
removable media, such as a memory card or magnetic disc.
3.7
gamma correction
signal processing operation that changes the relative signal levels
Note 1 to entry: Gamma correction is performed in part to correct for the nonlinear light output versus signal
input characteristics of the display. The relationship between the light input level and the output signal level,
called the camera opto-electronic conversion function (OECF), provides the gamma correction curve shape for an
image capture device.
Note 2 to entry: The gamma correction is usually an algorithm, lookup table, or circuit which operates separately
on each colour component of an image.
3.8
horizontal resolution
resolution value measured in the longer image dimension, corresponding to the horizontal direction for
a “landscape” image orientation, typically using a vertical or near vertical oriented test-chart feature
3.9
image aspect ratio
ratio of the image width to the image height
3.10
image compression
process that alters the way digital image data is encoded to reduce the size of an image file
3.11
image sensor
electronic device that converts incident electromagnetic radiation into an electronic signal
EXAMPLE Charge coupled device (CCD) array, complementary metal-oxide semiconductor (CMOS) array.
3.12
line pairs per millimetre
lp/mm
spatial frequency unit defined as the number of equal width black and white line pairs per millimetre
3.13
line spread function
LSF
normalized spatial signal distribution in the linearized output of an imaging system resulting from
imaging a theoretical infinitely thin line
3.14
line widths per picture height
LW/PH
spatial frequency unit for specifying the width of a feature on a test chart relative to the height of the
active area of the chart
Note 1 to entry: the value in LW/PH indicates the total number of lines of the same width which can be placed edge
to edge within the height of a test target or within the vertical field of view of a camera.
Note 2 to entry: This unit is used whatever the orientation of the “feature” (e.g. line). Specifically, it applies to
horizontal, vertical, and diagonal lines.
EXAMPLE If the height of the active area of the chart equals 20 cm, a black line of 1 000 LW/PH has a width
equal to 20/1 000 cm.
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3.15
linearized
digital signal conversion performed to invert the camera opto-electronic conversion function (OECF) to
focal plane exposure or scene luminance
3.16
lines per millimetre
lines/mm
spatial frequency unit defined as the number of equal width black and white lines per millimetre
Note 1 to entry: One line pair per millimetre (lp/mm) is equal to 2 lines/mm.
3.17
modulation
normalized amplitude of signal levels
Note 1 to entry: This is the difference between the minimum and maximum signal levels divided by the average
signal level.
3.18
modulation transfer function
MTF
modulus of the optical transfer function
Note 1 to entry: For the MTF to have significance, it is necessary that the imaging system be operating in an
isoplanatic region and in its linear range. Because most electronic still-picture cameras provide spatial colour
sampling and nonlinear processing, a meaningful MTF of the camera can only be approximated through the SFR.
See ISO 15529:2010.
3.19
normalized spatial frequency
spatial frequency unit for specifying resolution characteristics of an imaging system in terms of cycles
per pixel rather than in cycles/millimetre or any other unit of length
3.20
optical transfer function
OTF
two-dimensional Fourier transform of the imaging system’s point spread function
Note 1 to entry: For the OTF to have significance, it is necessary that the imaging system be operating in an
isoplanatic region and in its linear range. The OTF is a complex function whose modulus has unity value at zero
spatial frequency. (See ISO 9334). Because most electronic still-picture cameras provide spatial colour sampling
and nonlinear processing, a meaningful OTF of the camera can only be approximated through the SFR.
3.21
point spread function
PSF
normalized spatial signal distribution in the linearized output of an imaging system resulting from
imaging a theoretical infinitely small point source
3.22
resolution
measure of the ability of a camera system, or a component of a camera system, to depict picture detail
3.23
sampled imaging system
imaging system or device which generates an image signal by sampling an image at an array of discrete
points, or along a set of discrete lines, rather than a continuum of points
Note 1 to entry: The sampling at each point is done using a finite-size sampling aperture or area.
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3.24
spatial frequency response
SFR
relative amplitude response of an imaging system as a function of input spatial frequency
Note 1 to entry: The SFR is normally represented by a curve of the output response to an input sinusoidal spatial
luminance distribution of unit amplitude, over a range of spatial frequencies. The SFR is divided by its value at the
spatial frequency of 0 as normalization to yield a value of 1,0 at a spatial frequency of 0.
3.24.1
edge-based spatial frequency response
e-SFR
measured amplitude response of an imaging system to a slanted-edge input, as defined in Clause 6
3.24.2
sine wave-based spatial frequency response
s-SFR
measured amplitude response of an imaging system to a range of sine wave inputs, as defined in Clause 7
3.25
spectrally neutral
exhibiting reflective or transmissive characteristics which are constant over the wavelength range of
interest
3.26
test chart
arrangement of test patterns designed to test particular aspects of an imaging system
3.27
test pattern
specified arrangement of spectral reflectance or transmittance characteristics used in measuring an
image quality attribute
3.27.1
bi-tonal pattern
pattern that is spectrally neutral or effectively spectrally neutral, and consists exclusively of two
reflectance or transmittance values in a prescribed spatial arrangement
Note 1 to entry: Bi-tonal patterns are typically used to measure resolution by visual resolution method.
3.27.2
hyperbolic wedge test pattern
bi-tonal pattern that varies continuously and linearly with spatial frequency
Note 1 to entry: A bi-tonal hyperbolic wedge test pattern is used to measure resolution by the visual resolution
method in this International Standard.
3.28
vertical resolution
resolution value measured in the shorter image dimension, corresponding to the vertical direction for a
“landscape” image orientation, typically using a horizontal or near horizontal oriented test-chart feature
3.29
visual resolution
spatial frequency at which all of the individual black and white lines of a test pattern frequency can no
longer be distinguished by a human observer
Note 1 to entry: This presumes the features are reproduced on a display or print.
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4 Test conditions
4.1 Test chart illuminatio
...