SIST ISO 12233:2023:2023

Photography — Electronic still picture imaging — Resolution and spatial frequency responses

This document specifies methods for measuring the resolution and the spatial frequency response (SFR) of electronic still-picture cameras. It is applicable to the measurement of both monochrome and colour cameras which output digital data or analogue video signals.

Photographie — Imagerie des prises de vues électroniques — Résolution et réponses en fréquence spatiale

[Not translated]

General Information

• Status: Not Published
• ICS: 37.040.10 - Photographic equipment. Projectors
• Technical Committee: GRT - Graphical technology
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 17-Jun-2023
• Due Date: 22-Aug-2023
• Ref Project: ISO 12233:2023 - Photography — Electronic still picture imaging — Resolution and spatial frequency responses

Relations

• Revises: SIST ISO 12233:2017 - Photography - Electronic still picture imaging - Resolution and spatial frequency responses
• Effective Date: 23-Apr-2020

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 12233
Fourth edition
2023-02
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 12233:2023(E)
© ISO 2023
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ISO 12233:2023(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2023

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

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Published in Switzerland
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ISO 12233:2023(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction .............................................................................................................................................................................................................................. vi

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 ........................................................................................................... 6

4.3 Camera focusing .................................................................................................................................................................................... 6

4.4 Camera settings ..................................................................................................................................................................................... 6

4.5 White balance .......................................................................................................................................................................................... 7

4.6 Luminance and colour measurements .............................................................................................................................. 7

4.7 Gamma correction ............................................................................................................................................................................... 7

5 Visual resolution measurement ..........................................................................................................................................................7

5.1 General ........................................................................................................................................................................................................... 7

5.2 Test chart ..................................................................................................................................................................................................... 8

5.2.1 General ........................................................................................................................................................................................ 8

5.2.2 Material....................................................................................................................................................................................... 8

5.2.3 Size .................................................................................................................................................................................................. 8

5.2.4 Test patterns .......................................................................................................................................................................... 8

5.2.5 Test pattern modulation .............................................................................................................................................. 8

5.2.6 Positional tolerance ......................................................................................................................................................... 8

5.3 Rules of judgement for visual observation .................................................................................................................... 9

5.3.1 Rules of judgement ........................................................................................................................................................... 9

5.3.2 An example of a correct visual judgement .................................................................................................. 9

6 Edge-based spatial frequency response (e-SFR) .............................................................................................................10

6.1 General ........................................................................................................................................................................................................ 10

6.2 Methodology .......................................................................................................................................................................................... 13

7 Sinewave-based spatial frequency response (s-SFR) measurement .........................................................13

8 Presentation of results ...............................................................................................................................................................................14

8.1 General ........................................................................................................................................................................................................ 14

8.2 Resolution ................................................................................................................................................................................................ 15

8.2.1 General .....................................................................................................................................................................................15

8.2.2 Basic presentation ......................................................................................................................................................... 15

8.2.3 Representative presentation ................................................................................................................................ 15

8.3 Spatial frequency response (SFR) ...................................................................................................................................... 15

8.3.1 General .....................................................................................................................................................................................15

8.3.2 Spatial frequency response ................................................................................................................................... 15

8.3.3 Report of resolution value derived from the s-SFR .......................................................................... 16

Annex A (informative) CIPA resolution chart ..........................................................................................................................................18

Annex B (informative) Visual resolution measurement software ....................................................................................24

Annex C (informative) Edge SFR test chart ................................................................................................................................................30

Annex D (informative) Edge spatial frequency response (e-SFR) algorithm ........................................................32

Annex E (normative) Sine wave star test chart .....................................................................................................................................38

Annex F (normative) Sine wave spatial frequency response (s-SFR) analysis algorithm ........................41

Annex G (informative) Colour-filtered resolution measurements ...................................................................................46

Annex H (informative) Units and summary metrics .......................................................................................................................48

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ISO 12233:2023(E)

Annex I (informative) Original test chart defined in ISO 12233:2000 .........................................................................52

Annex J (informative) Non-uniform illumination compensation for some applications ..........................56

Annex K (informative) Derivation of correction functions ......................................................................................................62

Annex L (informative) Acutance calculation ............................................................................................................................................66

Annex M (informative) Matlab function for computing e-SFR ..............................................................................................69

Bibliography .............................................................................................................................................................................................................................75

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ISO 12233:2023(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 of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to

the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see

www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 42, Photography.

This fourth edition cancels and replaces the third edition (ISO 12233:2017), which has been technically

revised.
The main changes are as follows:

— In Clause 6 and Annex C, the e-SFR test chart has been modified by replacing the “slanted square”

features with four-cycle “slanted star” features, to enable diagonal measurements in addition to

horizontal and vertical measurements.

— In Clause 6 and Annex D, the e-SFR algorithm has been modified by using a Tukey window, by using

a 5th-order polynomial equation to fit the edge, and by correcting for the edge-angle sampling. As

a result, the measurement results may be slightly different compared to the results obtained using

the 3rd edition.
— Clause 6 and Annex D were updated to clarify the steps in the e-SFR algorithm.

— In Annex C, the reflectances of the surround and the light and dark patches were clarified.

— Annexes J, K, L, and M were added.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www.iso.org/members.html.
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ISO 12233:2023(E)
Introduction
Purpose

The spatial resolution capability is an important attribute of a digital camera. Resolution measurement

standards allow users to compare and verify spatial resolution measurements, as described in

Reference [14]. This document defines terminology, test charts, and test methods for performing

resolution measurements for analogue and digital cameras.
Technical background

Because digital cameras are sampled imaging systems, the term resolution is often incorrectly

interpreted as the number of addressable photoelements. While there are existing protocols for

determining camera pixel counts, these are not to be confused with the interpretation of resolution as

addressed in this document. 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. SFR is similar to the

optical transfer function (OTF) and the modulation transfer function (MTF) which are defined for linear

systems (see References [1] and [3]). 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 several 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 camera image processing, which can include image sharpening, 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 document, 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 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 spatial

frequencies. 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 document, two SFR measurements were described in the

second edition. The first SFR metrology method, an edge-based spatial frequency response (e-SFR), was

identical to that described in the first edition, except that a lower contrast edge was used for the test

chart. The test chart used for the e-SFR measurement has been updated in this fourth edition, to enable

measurements in diagonal directions. Regions of interest (ROIs) near slanted vertical, diagonal, and

horizontal edges are digitized and used to compute the e-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 e-SFR response.

A second sine wave based SFR (s-SFR) metrology method was introduced in the second 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 some consumer digital cameras. In this sense, it is intended to enable

easier interpretation of SFR levels from such cameras. 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 charts defined in this

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ISO 12233:2023(E)

document have been designed specifically to evaluate digital cameras. They have 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.

The measurements described in this document are performed using digital analysis techniques. They

are also applicable with analogue outputs of the camera by digitizing the analogue signals, if there is

adequate digitizing equipment.
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 document. While resolution metrology

of analogue 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 are

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 document, the test chart. 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 using the constrained camera

settings recommended in this document, these nonlinear operations can yield differing SFR results

depending on the test chart. Naturally, this causes confusion on which test charts 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 document. 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 test chart contrast edge features can be

significantly different, especially with respect to their morphology. This is largely due to nonlinear

image processing operations and would not occur for strictly linear imaging systems. To moderate

this behaviour, in the second edition of ISO 12233, a lower contrast slanted edge feature was chosen to

replace the higher contrast version of the first edition. The edge feature was further modified in this

fourth edition, to enable measurements in diagonal directions. This “slanted star” 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. However, data clipping is still possible

when using a test chart having a large edge reflectance ratio and/or when the captured image of the

test chart is significantly overexposed. This data clipping can cause the measured e-SFR values to be

overstated.

Sine wave features have long been the choice for directly calculating the MTF of analogue imaging

systems and they are intuitively satisfying. They were introduced in the second edition 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. Using the sine wave starburst test pattern (see Figure 6) adopted in the second

edition along 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.

Experience suggests that there is no single SFR for today’s digital cameras. Even under the strict

capture constraints suggested in this document, the allowable feature sets that most digital cameras

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ISO 12233:2023(E)

offer prevent such unique characterization. Confusion can be reduced through complete documentation

of the capture conditions and camera settings for which the SFR was calculated. It has been suggested

that comparing edge-based and sine wave-based SFR results under the same capture conditions

can be a good tool in assessing the contribution of spatial image processing in digital cameras. See

Reference [14].

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 measurement is also provided in this fourth edition of this document.

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 several measurement methods and how they relate to each other. These

results are provided in Reference [18].
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INTERNATIONAL STANDARD ISO 12233:2023(E)
Photography — Electronic still picture imaging —
Resolution and spatial frequency responses
1 Scope

This document specifies methods for measuring the resolution and the spatial frequency response

(SFR) of electronic still-picture cameras. It is applicable to the measurement of both monochrome and

colour cameras which output digital data or analogue video signals.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. 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.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
addressable photoelements
number of active photoelements in an image sensor (3.11)

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 (3.31) 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 (3.15) output of an imaging system resulting

from imaging a theoretical infinitely sharp edge
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ISO 12233:2023(E)
3.5
effectively spectrally neutral

having spectral characteristics which result in a specific imaging system producing the same output as

for a spectrally neutral (3.26) object

Note 1 to entry: Effectively spectrally neutral objects may have spectral reflectances or transmittances that

vary with wavelength (are not constant) so long as they produce a neutral response using the specified imaging

system. Objects that are effectively spectrally neutral with respect to one imaging system will not necessarily be

so with respect to another imaging system.
3.6
electronic still-picture camera

camera incorporating an image sensor (3.11) that outputs an analogue or digital signal representing a

still picture

Note 1 to entry: This camera may also record or store an analogue 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 (3.23) value(s) 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 are 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 (3.15) output of an imaging system resulting

from imaging a theoretical infinitely thin line
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