SIST ISO 12232:2020

INTERNATIONAL ISO
STANDARD 12232
Third edition
2019-02
Photography — Digital still cameras
— Determination of exposure index,
ISO speed ratings, standard output
sensitivity, and recommended
exposure index
Photographie — Appareils de prises de vue numériques —
Détermination de l'indice d'exposition, des régimes de vitesse ISO, de
la sensibilité normale de sortie et de l'indice d'exposition recommandé
Reference number
ISO 12232:2019(E)
©
ISO 2019

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ISO 12232:2019(E)

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© ISO 2019
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ISO 12232:2019(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Exposure index . 3
4.1 General . 3
4.2 Focal plane measurement . 4
4.3 Estimating focal plane exposure from scene luminance . 4
5 Test conditions . 4
5.1 General . 4
5.2 Illumination . 4
5.2.1 Daylight reference illuminant . 5
5.2.2 Tungsten reference illuminant . 5
5.3 Temperature and relative humidity . 5
5.4 White balance. 5
5.5 Infrared (IR) blocking filter. 5
5.6 Photosite integration time . 5
5.7 Compression . 5
5.8 Other DSC user settings . 5
6 Determination of ISO speed and ISO speed latitude . 6
6.1 General . 6
6.2 Saturation-based calculations. 6
6.2.1 Focal plane measurement . 6
6.2.2 Scene luminance measurement . 7
6.3 Noise-based calculations . 7
6.3.1 Focal plane method . . 7
6.3.2 Scene luminance method . 8
6.3.3 Colour cameras . . 8
6.3.4 Quantization effects . 8
6.4 Method of reporting . 9
7 Determination of standard output sensitivity (SOS) .11
7.1 Method for calculating SOS.11
7.2 Method of reporting .12
8 Specification of recommended exposure index (REI).12
8.1 General .12
8.2 Method for calculating recommended exposure index .12
8.3 Method of reporting .12
Annex A (informative) Recommended procedure for determining the noise-based ISO speed .15
Annex B (informative) Scene luminance and focal plane exposure .17
Annex C (informative) Recommended procedure for determining SOS values .19
Annex D (normative) Removing low frequency variations from the image data .20
Bibliography .21
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ISO 12232:2019(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 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 the following
URL: www .iso .org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 42, Photography.
This third edition cancels and replaces the second edition (ISO 12232:2006), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— definitions of photographic sensitivity and sensitivity setting were added;
— Clause 4 was simplified and clarified;
— it has been defined how to determine the exposure saturation level in 6.2;
— the applicability of SOS values for scene-dependent rendering and raw files was clarified;
— original Table 1 was expanded to include higher values, and separated the SOS and REI columns into
a new Table 2, which includes alternate values for some of the rows;
— Annex B was updated to clarify the mathematical basis of Formula (2);
— Annex E was cancelled.
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ISO 12232:2019(E)

Introduction
The exposure index (EI), ISO speed ratings, standard output sensitivity (SOS) and recommended
exposure index (REI) are useful metrics related to the capture of digital images. Standardization assists
users and manufacturers in determining the capabilities of Digital Still Cameras (DSCs), in setting DSCs
appropriately for the capture conditions, in obtaining well-exposed images, and in communicating
exposures and the related DSC characteristics in image files.
The exposures produced by a DSC are determined by the exposure time, the lens aperture, the lens
transmittance, the lens illumination falloff, the flare light present at the sensor, and the level and
spectral distribution of the scene radiances incident on the camera lens. However, it is not typical for
users to deal with this degree of complexity when determining and specifying image exposures. To
provide a means for simply communicating exposure information, this document specifies an exposure
index (EI) that corresponds to the focal plane exposure of a typical mid-tone. It is intended to be used
for setting the camera exposure and as a record of the camera exposure in image files.
When an image from a DSC is obtained using an insufficient exposure, proper tone reproduction can
generally be maintained by increasing the electronic or digital gain, but the image will contain an
unacceptable amount of noise. As the exposure is increased, the gain can be decreased, and, therefore,
the image noise can normally be reduced to an acceptable level. If the exposure is increased excessively,
the resulting signal in bright areas of the image may exceed the maximum signal level capacity of the
image sensor or camera signal processing. This can cause the image highlights to be clipped to form a
uniformly bright area, or to bloom into surrounding areas of the image. Therefore, it is important to
know the EI that will on-average produce the best image quality for specific DSC settings, and the range
of EIs over which the DSC can be expected to produce acceptable image quality. The ISO speed and
speed latitude ratings are intended to provide such information.
This document was designed to harmonize with earlier standards developed for film-based
photography. For example, the equations were chosen so that using a particular EI on a DSC should
result in approximately the same camera exposure settings, and resulting focal plane exposures, as
would be obtained using the same EI on a photographic film camera. For example, the value of 10 as
the constant in Formula (1) of this document is consistent with ISO 2721, so as to harmonize with this
earlier ISO standard for photographic film cameras. ISO 2721 uses the term nominal exposure and
assumes that the nominal exposure is an arithmetic mean exposure value, which usually corresponds
to the mid-tone in photographs of average scenes.
However, there are differences between electronic and film-based imaging systems that preclude exact
equivalency. DSCs can include variable gain and can provide digital processing after the image data has
been captured, enabling desired tone reproduction to be achieved over a range of camera exposures.
It is therefore possible for DSCs to have a range of ISO speed ratings. This range is defined as the ISO
speed latitude. To prevent confusion, a single value is designated as the ISO speed, with the ISO speed
latitude upper and lower limits indicating the speed range.
It can also be useful to compare or record the sensitivity of a DSC, for cases where the DSC has a fixed
sensitivity. The standard output sensitivity (SOS) is designed to meet this need. Likewise, it can be
useful to know the EI recommended by the DSC manufacturer for a specific condition. This information
is provided by the recommended exposure index (REI).
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INTERNATIONAL STANDARD ISO 12232:2019(E)
Photography — Digital still cameras — Determination
of exposure index, ISO speed ratings, standard output
sensitivity, and recommended exposure index
1 Scope
This document specifies the method for assigning and reporting ISO speed ratings, ISO speed latitude
ratings, standard output sensitivity values, and recommended exposure index values, for digital still
cameras. It is applicable to both monochrome and colour digital still cameras.
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 7589:2002, Photography — Illuminants for sensitometry — Specifications for daylight, incandescent
tungsten and printer
ISO 14524, Photography — Electronic still-picture cameras — Methods for measuring opto-electronic
conversion functions (OECFs)
ITU-R BT.709, Parameter values for the HDTV standards for production and international programme
exchange
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
digital still camera
DSC
device which incorporates an image sensor and which produces digital image data representing a
still picture
Note 1 to entry: A digital still camera is typically a portable, hand-held device. The device can provide additional
functions, for example video image capture or wireless telephony. The digital image data is usually recorded on a
removable memory, such as a solid-state memory card or using internal memory.
3.2
DSC image signal
image data stored or output by a digital still camera
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ISO 12232:2019(E)

3.3
exposure index
EI
numerical value that is inversely proportional to the exposure provided to an image sensor to obtain
an image
Note 1 to entry: Images obtained from a DSC using a range of exposure index values will normally provide a
range of image quality levels.
3.4
exposure saturation
minimum focal plane exposure that produces the maximum valid (not clipped or bloomed) DSC
image signal
Note 1 to entry: The exposure saturation is expressed in lux-seconds (lx·s).
3.5
exposure series
series of images of the same subject taken using different exposure index values
3.6
image sensor
electronic device that converts incident electromagnetic radiation into an electronic signal
EXAMPLE A charge coupled device (CCD) array or a CMOS image array.
3.7
ISO speed
numerical value calculated from the exposure provided at the focal plane of a DSC to produce specified
DSC image signal characteristics
Note 1 to entry: The ISO speed is usually the highest exposure index value that still provides peak image quality
for normal scenes. However, a DSC does not necessarily use the ISO speed value as the exposure index value when
capturing images.
3.8
ISO speed latitude
set of two numerical values calculated from the exposure provided at the focal plane of a DSC to produce
specified DSC image signal characteristics
Note 1 to entry: The ISO speed latitude is expected to correlate with the range of exposure index values that
provide acceptable image quality for normal scenes.
3.9
photographic sensitivity
general term used for numerical values calculated based on the exposure at the focal plane of a
DSC which produces a specified DSC image signal level, such as the standard output sensitivity or
recommended exposure index
Note 1 to entry: In practise, the photographic sensitivity is often called the "sensitivity" or the "camera
sensitivity". It is sometimes called the “ISO sensitivity”, for historical reasons that date from ISO standards for
photographic film cameras.
3.10
photosite integration time
total time period during which the photosites of an image sensor are able to integrate the light from the
scene to form an image
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ISO 12232:2019(E)

3.11
recommended exposure index
REI
specific exposure index value recommended by a DSC provider as a reference for adjusting photographic
accessories
Note 1 to entry: REI provides a practical exposure index value for setting the reference exposure index of light
meters, studio lighting, etc., but images taken using this exposure index value do not necessarily provide the best
image quality.
3.12
signal processing
operations performed by electronic circuits or algorithms that convert or modify the output of an
image sensor
3.13
sensitivity setting
numerical value of the photographic sensitivity used by a DSC when capturing images
Note 1 to entry: In some cases the sensitivity setting is set by the user. In other cases it is set automatically by
the DSC.
Note 2 to entry: In DSCs employing an automatic exposure control system, the difference between the EI value
used to capture an image and the sensitivity setting is called the “exposure bias”. The value of the exposure bias
is typically indicated using EV (exposure value) units.
Note 3 to entry: For historical reasons, the sensitivity setting of a DSC is often labelled the "ISO".
3.14
standard output sensitivity
SOS
specific exposure index value for a DSC that provides a still image with a specified DSC image signal
value under specified test conditions
Note 1 to entry: SOS provides a practical exposure index value based on the signal level of images captured with
a DSC, but images taken using this exposure index value do not necessarily provide the best image quality.
4 Exposure index
4.1 General
An exposure index (EI) is a numerical value that is inversely proportional to the exposure provided
to an image sensor to obtain an image. Images obtained from a DSC using a range of EI values will
normally provide a range of image quality levels. The photographic sensitivity of a DSC is a particular
EI value calculated from the exposure provided at the focal plane of the DSC that produces a specified
camera image signal level.
The EI value I for a specific image captured by a DSC shall be equal to the EI reference exposure of
EI
10 lx⋅s divided by the focal plane exposure used to capture the image, as specified in Formula (1):
IK= H (1)
EI m
where
K is a constant equal to 10 lx⋅s and
H is the average focal plane exposure, expressed in lx⋅s.
m
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ISO 12232:2019(E)

NOTE 1 For an average scene, the average focal plane exposure is a mid-tone exposure, which is approximately
equal to the exposure which would be obtained from an 18 % reflectance test card positioned in the scene and
illuminated by the main light source.
In the case where the image recorded by the camera is the result of processing that combines multiple
captured images, the EI value corresponds to the sum of the exposures for the images that have been
combined. In this case, the EI value should be reported along with a notation indicating that multiple
images have been combined.
EI values should be reported using exposure index metadata in the image file header. In the case where
multiple captured images have been combined, the number of captured images that were combined and
the EI values for the individual captured images should also be reported using metadata.
4.2 Focal plane measurement
For DSCs where it is possible to measure the focal plane exposure directly, such as a DSC with a
removable lens, the value of H should be measured using spatially uniform and spectrally neutral
m
illumination. The measurement should be made within a circle lying in the centre of the focal plane
with a diameter of 75/100 times the shorter dimension of the image field.
4.3 Estimating focal plane exposure from scene luminance
For DSCs where it is not practical to measure the focal plane exposure, the value of H required in
m
Formula (1) should be estimated using Formula (2). The measurement should be made using a test card
for a flat-field image (i.e. a spatially uniform field-of-view of spectrally neutral reflectance) at a distance
of at least one meter, using only the portion of the field of view of the image sensor that is subtended
by an angle of less than 10°. If the lens focus can be manually adjusted, it should be set to the infinity
position. The derivation of Formula (2) is given in Annex B.
65Lt
a
H = (2)
m
2
100A
where
A is the f -number of the lens;
L is the luminance, expressed in candelas per square metre, measured using a full frame uniformly
a
illuminated diffuse reflecting test card (i.e. a spatially uniform field-of-view of spectrally neutral
reflectance );
t is the photosite integration time, expressed in seconds.
5 Test conditions
5.1 General
The following measurement conditions should be used as nominal conditions when determining the ISO
speed ratings, SOS, and REI values of a DSC. If it is not possible or not appropriate to use these nominal
operating conditions, the actual operating conditions shall be listed along with the reported values.
5.2 Illumination
The reported values shall indicate whether illumination approximating the ISO 7589 daylight or studio
tungsten illuminant was used. ISO 7589 describes the procedures for determining if the illumination
used in a specific speed rating determination test is an acceptable match to the daylight and studio
tungsten illuminants. Also, the spectral power distribution of the illumination used should be
reasonably similar to that of the ISO 7589 daylight or studio tungsten reference illuminant.
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ISO 12232:2019(E)

5.2.1 Daylight reference illuminant
For daylight measurements without the camera lens, the ISO sensitometric daylight illuminant given
in ISO 7589:2002, Table 1, shall be used. This illuminant is defined as the product of the spectral
power distribution of CIE colorimetric standard illuminant D and the spectral transmittance of the
55
International Standard camera lens. For measurements with the camera lens in place, the spectral
radiance characteristics of the light used for the measurement should be equivalent to the daylight ISO
standard source provided in the second column of ISO 7589:2002, Table 1. In order to apply the ISO SDI
(spectral distribution index) criterion, the spectral radiance of the light shall be measured and then
multiplied by the relative spectral transmittance of the ISO standard lens, which is also described in
ISO 7589, prior to multiplying by the weighted spectral sensitivities.
5.2.2 Tungsten reference illuminant
For tungsten measurements without the camera lens, the ISO sensitometric studio tungsten illuminant
given in ISO 7589:2002, Table 2, shall be used. This illuminant is defined as the product of the average
spectral power distribution of experimentally measured sources having a colour temperature of
approximately 3 050 K and the spectral transmittance of the International Standard camera lens. For
measurements with the camera lens in place, the spectral radiance characteristics of the light used for
the measurement should be equivalent to the tungsten ISO standard source provided in the second
column of ISO 7589:2002, Table 2. In order to apply the ISO SDI (spectral distribution index) criterion,
the spectral radiance of the light shall be measured and then multiplied by the relative spectral
transmittance of the ISO standard lens, which is also described in ISO 7589, prior to multiplying by the
weighted spectral sensitivities.
5.3 Temperature and relative humidity
The ambient temperature during the acquisition of the test data shall be (23 ± 2) °C and the relative
humidity should be (50 ± 20) %.
5.4 White balance
For a colour camera, the camera white balance should be adjusted, if possible, to provide proper white
balance (equal RGB signal levels) for the illumination light source, as specified in ISO 14524.
5.5 Infrared (IR) blocking filter
If required, an infrared (IR) blocking filter shall be used as specified in ISO 14524.
5.6 Photosite integration time
The photosite integration time should not be longer than 1/30 s.
5.7 Compression
If the DSC includes any form of lossy compression, the compression shall be disabled, if possible, during
the determination of σ(D ) or σ(D ) in Clause 6. If it is not possible to disable the camera compression,
H L
the noise-based values cannot be properly determined, and shall not be reported.
5.8 Other DSC user settings
All other camera controls (e.g. sharpness, contrast) shall be set to the factory default settings. Additional,
optional, measurements can also be made using camera control settings that are not the factory default
settings. However, the reporting of such optional measurements shall be done in a manner that does not
cause confusion with the primary measurements made using the factory default settings.
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ISO 12232:2019(E)

6 Determination of ISO speed and ISO speed latitude
6.1 General
With appropriate electrical or digital gain, a DSC can provide an appropriate DSC image signal level
for a range of sensor exposure levels. The maximum exposure level is just below the exposure le
...