SIST ISO 12232:2011
Photography - Digital still cameras - Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index
Photography - Digital still cameras - Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index
This International Standard 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. This International Standard is applicable to both monochrome and colour digital still cameras.
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
Fotografija - Digitalne kamere za mirujoče slike - Določanje indeksa ekspozicije, splošnih občutljivosti ISO, standardne izhodne občutljivosti in priporočenega indeksa ekspozicije
Ta mednarodni standard določa metodo za dodeljevanje in poročanje o splošnih občutljivostih ISO, kotnih občutljivostih ISO, vrednostih standardne izhodne občutljivosti in vrednostih priporočenega indeksa ekspozicije digitalnih kamer za mirujoče slike. Ta mednarodni standard velja za črno-bele in barvne digitalne kamere za mirujoče slike.
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INTERNATIONAL ISO
STANDARD 12232
Second edition
2006-04-15
Corrected version
2006-10-01
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 12232:2006(E)
©
ISO 2006
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ISO 12232:2006(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2006
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2006 – All rights reserved
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ISO 12232:2006(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Exposure index values.3
4.1 General.3
4.2 Focal plane measurement.3
4.3 Scene luminance measurement.3
5 Test conditions .4
5.1 General.4
5.2 Illumination.4
5.2.1 Daylight illumination .4
5.2.2 Tungsten illumination .4
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.5
6.1 General.5
6.2 Saturation-based speed .6
6.2.1 Focal plane measurement.6
6.2.2 Scene luminance measurement.6
6.3 Noise-based speed .6
6.3.1 Focal plane method .6
6.3.2 Scene luminance method .7
6.3.3 Colour cameras.7
6.3.4 Quantization effects.8
6.4 Method of reporting.8
7 Determination of standard output sensitivity (SOS).8
7.1 Method for calculating SOS.9
7.2 Method of reporting.9
8 Specification of recommended exposure index (REI) .9
8.1 Method for calculating recommended exposure index .9
8.2 Method of reporting.10
Annex A (informative) Recommended procedure for determining the noise-based ISO speed.11
Annex B (informative) Scene luminance and focal plane exposure .13
Annex C (informative) Recommended procedure for determining SOS values .14
Annex D (normative) Removing low frequency variations from the image data.15
Annex E (informative) Calculation of noise-based minimum illumination level.16
Bibliography .17
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ISO 12232:2006(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. 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 document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 12232 was prepared by Technical Committee ISO/TC 42, Photography.
This second edition cancels and replaces the first edition (ISO 12232:1998), which has been technically
revised.
This corrected version incorporates the following corrections:
⎯ the normative reference ISO 7589 has been dated;
⎯ the symbol for the effective f-number of the lens has been made consistent in Equations (2), (3) and (4);
⎯ the cross-references in the column headings have been corrected in Table 1;
⎯ Figure A.1 has been changed and notes and footnotes have been added for better clarity;
⎯ Equation (B.1) has been corrected and the symbol for the vignetting factor changed;
⎯ the second paragraph in Annex D has been reworded and changed to a note to reflect its intentional
informative nature;
⎯ the second sentence in Table D.1 has been slightly reworded and added at the end of the paragraph
preceding Table D.1;
⎯ in Table D.1, zeros have been added to values to improve their readability and the text below the values
has been changed to Note 1 to show its intentional informative nature;
⎯ a note has been added to both Figure A.1 and Table D.1 to notify the reader that the decimal sign is a
comma in accordance with ISO 31-0;
⎯ ISO 31-0 has been added to the Bibliography and the references have been renumbered accordingly.
iv © ISO 2006 – All rights reserved
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ISO 12232:2006(E)
Introduction
The ISO speed rating, standard output sensitivity (SOS) and recommended exposure index (REI) are
important attributes of digital still cameras (DSCs). Standardization assists users and manufacturers in
obtaining proper exposures and in determining the low light capability of DSCs.
The exposure level of a DSC is determined by the exposure time, the lens aperture, the lens transmittance,
the level and spectral distribution of the scene illumination, and the scene reflectance. 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 guide the user in setting proper exposures. An ISO speed
rating is intended to serve as such a guide. The methods for assigning an ISO speed rating to a DSC
harmonize with current film-based photographic standards. In order to be easily understood by photographers,
the ISO speed rating for a DSC should directly relate to the ISO speed rating for photographic film cameras.
For example, if a DSC has an ISO speed rating of ISO 100, then the same exposure time and aperture should
be appropriate for an ISO 100 rated film/process system.
The ISO speed ratings described in this International Standard are intended to harmonize with film ISO speed
ratings. 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 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.
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INTERNATIONAL STANDARD ISO 12232:2006(E)
Photography — Digital still cameras — Determination of
exposure index, ISO speed ratings, standard output sensitivity,
and recommended exposure index
1 Scope
This International Standard 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. This International Standard is applicable to both monochrome and colour digital still cameras.
2 Normative references
The following referenced documents are indispensable for the application 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 554, Standard atmospheres for conditioning and/or testing — Specifications
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)
IEC 61966-2-1, Multimedia systems and equipment — Colour measurement and management — Part 2-1:
Colour management — Default RGB colour space — sRGB
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.
3.1
digital still camera
DSC
device which incorporates an image sensor and which produces a digital signal representing a still picture
NOTE A digital still camera is typically a portable, hand-held device. The digital signal is usually recorded on a
removable memory, such as a solid-state memory card or magnetic disk.
3.2
exposure index
EI
numerical value that is inversely proportional to the exposure provided to an image sensor to obtain an image
NOTE Images obtained from a DSC using a range of exposure index values will normally provide a range of image
quality levels.
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ISO 12232:2006(E)
3.3
exposure series
series of images of the same subject taken using different exposure index values
3.4
image sensor
electronic device that converts incident electromagnetic radiation into an electronic signal
EXAMPLE A charge coupled device (CCD) array.
3.5
ISO speed
numerical value calculated from the exposure provided at the focal plane of a DSC to produce specified
camera output signal characteristics using the methods described in this International Standard
NOTE 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.
3.6
ISO speed latitude
set of two numerical values calculated from the exposure provided at the focal plane of a DSC to produce
specified camera output signal characteristics using the methods described in this International Standard
NOTE The ISO speed latitude is expected to correlate with the range of exposure index values that provide
acceptable image quality for normal scenes.
3.7
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
3.8
recommended exposure index
REI
specific exposure index value recommended by a DSC provider as a reference for adjusting photographic
accessories, as defined in this International Standard
NOTE 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.
3.9
signal processing
operations performed by electronic circuits or algorithms that convert or modify the output of an image sensor
3.10
standard output sensitivity
SOS
specific exposure index value for a DSC that provides a still image with a specified digital output signal value
under specified test conditions, as defined in this International Standard
NOTE 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.
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ISO 12232:2006(E)
4 Exposure index values
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 ISO speed of a DSC is equal to a particular exposure index value
calculated from the exposure provided at the focal plane of the DSC to produce specified camera output
signal characteristics, using the methods described in this International Standard. The equations used in this
International Standard have been chosen to create a link between electronic and conventional silver-halide-
based photographic systems. Using a particular ISO speed value as the exposure index on a DSC should
result in the same camera exposure settings, and resulting focal plane exposures, as would be obtained using
the same exposure index on a film camera or other photographic exposure meter.
Where possible, the exposure index values corresponding to the arithmetic mean focal plane exposure used
to capture an image should be reported in the image file header as the exposure index.
4.2 Focal plane measurement
For DSC exposure meters, where the arithmetic mean focal plane exposure is measured within a circle lying
in the centre of the image with a diameter of 75/100 times the shorter dimension of the image field, the
exposure index values, I , should be computed using Equation (1), as described in ISO 2721.
EI
I = 10/H (1)
EI a
where H is the arithmetic mean focal plane exposure, expressed in lux-seconds (lx⋅s).
a
NOTE 1 The value of 10 as the constant in Equation (1) is consistent with ISO 2721 and ISO 5763. These International
Standards assume that the exposure is an arithmetic mean value, as is normally provided by a camera light meter. If the
geometric mean exposure was used in place of the arithmetic mean exposure, a lower value for this constant would be
appropriate. Note that the arithmetic mean exposure is obtained when the linear exposure values are averaged, while a
geometric mean exposure is obtained by taking the antilog of the average of the logarithmic exposure values. An
approximation to the geometric mean is also obtained by taking the antilog of the average measured film densities in
conventional photographic systems, provided that the film H&D curve has a straight line characteristic over the film
exposure range. Note also that the brightness response of the human visual system to the luminances of objects in a
scene is approximately logarithmic.
NOTE 2 The arithmetic mean focal plane exposure for statistically average scenes is often assumed to be equal to
approximately 18 % of the focal plane exposure, which would be obtained from a perfectly diffuse 100 % reflectance
object in a statistically average scene. Therefore, the arithmetic mean focal plane exposure would equal 2/10 times the
focal plane exposure that would be obtained from a 90 % reflectance test card in a statistically average scene.
4.3 Scene luminance measurement
For DSC exposure meters where the arithmetic mean scene luminance is measured, the expected value of
the arithmetic mean focal plane exposure, H , required in Equation (1) can be computed using Equation (2).
a
The derivation of Equation (2) is given in Annex B.
65L t
a
H = (2)
a
2
100A
eff
where
A is the effective f-number of the lens;
eff
L is the arithmetic mean luminance, expressed in candelas per square metre;
a
t is the photosite integration time, expressed in seconds.
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ISO 12232:2006(E)
NOTE The laboratory measurement of L can be simplified by using a full frame uniformly illuminated diffuse reflecting
test card, so that the arithmetic mean luminance can be measured by simply measuring the luminance at the centre of the
image.
The effective f-number, A , of the lens for the focused image shall be calculated using Equation (3):
eff
A = (1 + 1/R) f (3)
eff #
where
R is the ratio of the height of the camera field of view at the focus distance to the height of the image at
the focal plane;
f is the f-number of the lens.
#
If the camera is focused at infinity, the effective f-number is equal to the f-number of the lens.
Therefore, for electronic still (or other) camera exposure meters where the arithmetic mean scene luminance
is measured, exposure index values should be computed using Equation (4), derived by substituting
Equation (2) into Equation (1).
2
154A
eff
I = (4)
EI
10L t
a
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 appropriate to achieve 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 the daylight or 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 tungsten sensitometric illuminants.
5.2.1 Daylight illumination
For daylight measurements without the camera lens, the ISO sensitometric daylight illuminant given in Table 1
of ISO 7589:2002 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 International Standard camera
55
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 Table 1 of ISO 7589:2002. 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 illumination
For tungsten measurements without the camera lens, the ISO sensitometric studio tungsten illuminant given
in Table 2 of ISO 7589:2002 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
4 © ISO 2006 – All rights reserved
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ISO 12232:2006(E)
to the tungsten ISO standard source provided in the second column of Table 2 of ISO 7589:2002. 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, as specified in ISO 554,
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, the
H L
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, for example with the DSC set to monochrome mode. 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.
6 Determination of ISO speed
6.1 General
With appropriate electrical or digital gain, a DSC can provide an appropriate output signal level for a range of
sensor exposure levels. The maximum exposure level is the exposure level where typical picture highlights
will be clipped as a result of saturating the image sensor signal capacity or reaching the camera signal
processing maximum signal level. The minimum exposure level depends on the amount of noise that can be
tolerated in the image. These situations lead to two different types of speed values, saturation signal-based
values and noise-based values. The ISO speed is preferably determined using a noise-based method. The
saturation-based value is preferably used to indicate the camera's overexposure speed latitude. A second
noise-based value is preferably used to indicate the camera's underexposure speed latitude. For some types
of DSCs, such as those employing lossy compression methods, it is not possible to correctly determine the
noise-based ISO speed. In such cases, the ISO speed of the camera is determined using the saturation-
based measurement, and the ISO speed latitude values are not reported. In other cases, the noise-based ISO
speed may be lower than the saturation-based speed, in which case the saturation based-speed is reported.
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ISO 12232:2006(E)
6.2 Saturation-based speed
In photographic applications where the scene illumination level can be controlled, for example in studio
photography, the photographer normally prefers to use a camera exposure index which provides the best
possible image quality. In this situation, a saturation signal-based rating is appropriate. This rating allows the
user to set the camera exposure so that typical image highlights are just below the maximum possible
(saturation) camera signal value.
6.2.1 Focal plane measurement
The saturation based speed, S , of an electronic still picture camera is defined as:
sat
S = 78/H (5)
sat sat
where H is the minimum focal plane exposure, expressed in lux-seconds (lx⋅s), that produces the maximum
sat
valid (not clipped or bloomed) camera output signal.
NOTE Equation (5) provides 1/2 “stop" of headroom (41 % additional headroom) for specular highlights above the
signal level that would be obtained from a theoretical 100 % reflectance object in the scene, so that a theoretical 141 %
reflectance object in the scene would produce a focal plane exposure of H . Therefore, an 18 % reflectance test card in
sat
the scene would produce a focal plane exposure of 128/1 000 H . Thus, the multiplicative constant 78 in Equation (5) is
sat
equal to 10 times 1 000/128, where the value 10 is the constant from Equation (1).
6
...
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.ULSRURþHQHJDPhotographie - 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 recommandéPhotography - Digital still cameras - Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index37.040.10Fotografska oprema. ProjektorjiPhotographic equipment. ProjectorsICS:Ta slovenski standard je istoveten z:ISO 12232:2006SIST ISO 12232:2011en01-julij-2011SIST ISO 12232:2011SLOVENSKI
STANDARD
SIST ISO 12232:2011
Reference numberISO 12232:2006(E)© ISO 2006
INTERNATIONAL STANDARD ISO12232Second edition2006-04-15Corrected version2006-10-01Photography — 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
SIST ISO 12232:2011
ISO 12232:2006(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this area. Adobe is a trademark of Adobe Systems Incorporated. Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
©
ISO 2006 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO's member body in the country of the requester. ISO copyright office Case postale 56 • CH-1211 Geneva 20 Tel.
+ 41 22 749 01 11 Fax
+ 41 22 749 09 47 E-mail
copyright@iso.org Web
www.iso.org Published in Switzerland
ii
© ISO 2006 – All rights reserved
SIST ISO 12232:2011
ISO 12232:2006(E) © ISO 2006 – All rights reserved
iiiContents Page Foreword.iv Introduction.v 1 Scope.1 2 Normative references.1 3 Terms and definitions.1 4 Exposure index values.3 4.1 General.3 4.2 Focal plane measurement.3 4.3 Scene luminance measurement.3 5 Test conditions.4 5.1 General.4 5.2 Illumination.4 5.2.1 Daylight illumination.4 5.2.2 Tungsten illumination.4 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.5 6.1 General.5 6.2 Saturation-based speed.6 6.2.1 Focal plane measurement.6 6.2.2 Scene luminance measurement.6 6.3 Noise-based speed.6 6.3.1 Focal plane method.6 6.3.2 Scene luminance method.7 6.3.3 Colour cameras.7 6.3.4 Quantization effects.8 6.4 Method of reporting.8 7 Determination of standard output sensitivity (SOS).8 7.1 Method for calculating SOS.9 7.2 Method of reporting.9 8 Specification of recommended exposure index (REI).9 8.1 Method for calculating recommended exposure index.9 8.2 Method of reporting.10 Annex A (informative)
Recommended procedure for determining the noise-based ISO speed.11 Annex B (informative)
Scene luminance and focal plane exposure.13 Annex C (informative)
Recommended procedure for determining SOS values.14 Annex D (normative)
Removing low frequency variations from the image data.15 Annex E (informative)
Calculation of noise-based minimum illumination level.16 Bibliography.17
SIST ISO 12232:2011
ISO 12232:2006(E) iv
© ISO 2006 – All rights reserved 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 2. The main task of technical committees is to prepare International Standards. 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 document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 12232 was prepared by Technical Committee ISO/TC 42, Photography. This second edition cancels and replaces the first edition (ISO 12232:1998), which has been technically revised. This corrected version incorporates the following corrections: ⎯ the normative reference ISO 7589 has been dated; ⎯ the symbol for the effective f-number of the lens has been made consistent in Equations (2), (3) and (4); ⎯ the cross-references in the column headings have been corrected in Table 1; ⎯ Figure A.1 has been changed and notes and footnotes have been added for better clarity; ⎯ Equation (B.1) has been corrected and the symbol for the vignetting factor changed; ⎯ the second paragraph in Annex D has been reworded and changed to a note to reflect its intentional informative nature; ⎯ the second sentence in Table D.1 has been slightly reworded and added at the end of the paragraph preceding Table D.1; ⎯ in Table D.1, zeros have been added to values to improve their readability and the text below the values has been changed to Note 1 to show its intentional informative nature; ⎯ a note has been added to both Figure A.1 and Table D.1 to notify the reader that the decimal sign is a comma in accordance with ISO 31-0; ⎯ ISO 31-0 has been added to the Bibliography and the references have been renumbered accordingly. SIST ISO 12232:2011
ISO 12232:2006(E) © ISO 2006 – All rights reserved
vIntroduction The ISO speed rating, standard output sensitivity (SOS) and recommended exposure index (REI) are important attributes of digital still cameras (DSCs). Standardization assists users and manufacturers in obtaining proper exposures and in determining the low light capability of DSCs. The exposure level of a DSC is determined by the exposure time, the lens aperture, the lens transmittance, the level and spectral distribution of the scene illumination, and the scene reflectance. 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 guide the user in setting proper exposures. An ISO speed rating is intended to serve as such a guide. The methods for assigning an ISO speed rating to a DSC harmonize with current film-based photographic standards. In order to be easily understood by photographers, the ISO speed rating for a DSC should directly relate to the ISO speed rating for photographic film cameras. For example, if a DSC has an ISO speed rating of ISO 100, then the same exposure time and aperture should be appropriate for an ISO 100 rated film/process system. The ISO speed ratings described in this International Standard are intended to harmonize with film ISO speed ratings. 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 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.
SIST ISO 12232:2011
SIST ISO 12232:2011
INTERNATIONAL STANDARD ISO 12232:2006(E) © ISO 2006 – All rights reserved
1Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index 1 Scope This International Standard 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. This International Standard is applicable to both monochrome and colour digital still cameras. 2 Normative references The following referenced documents are indispensable for the application 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 554, Standard atmospheres for conditioning and/or testing — Specifications 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) IEC 61966-2-1, Multimedia systems and equipment — Colour measurement and management — Part 2-1: Colour management — Default RGB colour space — sRGB 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. 3.1 digital still camera DSC device which incorporates an image sensor and which produces a digital signal representing a still picture NOTE A digital still camera is typically a portable, hand-held device. The digital signal is usually recorded on a removable memory, such as a solid-state memory card or magnetic disk. 3.2 exposure index EI numerical value that is inversely proportional to the exposure provided to an image sensor to obtain an image NOTE Images obtained from a DSC using a range of exposure index values will normally provide a range of image quality levels. SIST ISO 12232:2011
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© ISO 2006 – All rights reserved 3.3 exposure series series of images of the same subject taken using different exposure index values 3.4 image sensor electronic device that converts incident electromagnetic radiation into an electronic signal EXAMPLE A charge coupled device (CCD) array. 3.5 ISO speed numerical value calculated from the exposure provided at the focal plane of a DSC to produce specified camera output signal characteristics using the methods described in this International Standard NOTE 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. 3.6 ISO speed latitude set of two numerical values calculated from the exposure provided at the focal plane of a DSC to produce specified camera output signal characteristics using the methods described in this International Standard NOTE The ISO speed latitude is expected to correlate with the range of exposure index values that provide acceptable image quality for normal scenes. 3.7 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 3.8 recommended exposure index REI specific exposure index value recommended by a DSC provider as a reference for adjusting photographic accessories, as defined in this International Standard NOTE 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. 3.9 signal processing operations performed by electronic circuits or algorithms that convert or modify the output of an image sensor 3.10 standard output sensitivity SOS specific exposure index value for a DSC that provides a still image with a specified digital output signal value under specified test conditions, as defined in this International Standard NOTE 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. SIST ISO 12232:2011
ISO 12232:2006(E) © ISO 2006 – All rights reserved
34 Exposure index values 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 ISO speed of a DSC is equal to a particular exposure index value calculated from the exposure provided at the focal plane of the DSC to produce specified camera output signal characteristics, using the methods described in this International Standard. The equations used in this International Standard have been chosen to create a link between electronic and conventional silver-halide-based photographic systems. Using a particular ISO speed value as the exposure index on a DSC should result in the same camera exposure settings, and resulting focal plane exposures, as would be obtained using the same exposure index on a film camera or other photographic exposure meter. Where possible, the exposure index values corresponding to the arithmetic mean focal plane exposure used to capture an image should be reported in the image file header as the exposure index. 4.2 Focal plane measurement For DSC exposure meters, where the arithmetic mean focal plane exposure is measured within a circle lying in the centre of the image with a diameter of 75/100 times the shorter dimension of the image field, the exposure index values, IEI, should be computed using Equation (1), as described in ISO 2721. IEI = 10/Ha (1) where Ha is the arithmetic mean focal plane exposure, expressed in lux-seconds (lx⋅s). NOTE 1 The value of 10 as the constant in Equation (1) is consistent with ISO 2721 and ISO 5763. These International Standards assume that the exposure is an arithmetic mean value, as is normally provided by a camera light meter. If the geometric mean exposure was used in place of the arithmetic mean exposure, a lower value for this constant would be appropriate. Note that the arithmetic mean exposure is obtained when the linear exposure values are averaged, while a geometric mean exposure is obtained by taking the antilog of the average of the logarithmic exposure values. An approximation to the geometric mean is also obtained by taking the antilog of the average measured film densities in conventional photographic systems, provided that the film H&D curve has a straight line characteristic over the film exposure range. Note also that the brightness response of the human visual system to the luminances of objects in a scene is approximately logarithmic. NOTE 2 The arithmetic mean focal plane exposure for statistically average scenes is often assumed to be equal to approximately 18 % of the focal plane exposure, which would be obtained from a perfectly diffuse 100 % reflectance object in a statistically average scene. Therefore, the arithmetic mean focal plane exposure would equal 2/10 times the focal plane exposure that would be obtained from a 90 % reflectance test card in a statistically average scene. 4.3 Scene luminance measurement For DSC exposure meters where the arithmetic mean scene luminance is measured, the expected value of the arithmetic mean focal plane exposure, Ha, required in Equation (1) can be computed using Equation (2). The derivation of Equation (2) is given in Annex B. aa2eff65100LtHA= (2) where Aeff is the effective f-number of the lens; La is the arithmetic mean luminance, expressed in candelas per square metre; t is the photosite integration time, expressed in seconds. SIST ISO 12232:2011
ISO 12232:2006(E) 4
© ISO 2006 – All rights reserved NOTE The laboratory measurement of L can be simplified by using a full frame uniformly illuminated diffuse reflecting test card, so that the arithmetic mean luminance can be measured by simply measuring the luminance at the centre of the image. The effective f-number, Aeff, of the lens for the focused image shall be calculated using Equation (3): Aeff = (1 + 1/R) f# (3) where
R is the ratio of the height of the camera field of view at the focus distance to the height of the image at the focal plane; f# is the f-number of the lens. If the camera is focused at infinity, the effective f-number is equal to the f-number of the lens. Therefore, for electronic still (or other) camera exposure meters where the arithmetic mean scene luminance is measured, exposure index values should be computed using Equation (4), derived by substituting Equation (2) into Equation (1). eff2EIa15410AILt= (4) 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 appropriate to achieve 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 the daylight or 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 tungsten sensitometric illuminants. 5.2.1 Daylight illumination For daylight measurements without the camera lens, the ISO sensitometric daylight illuminant given in Table 1 of ISO 7589:2002 shall be used. This illuminant is defined as the product of the spectral power distribution of CIE colorimetric standard illuminant D55 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 daylight ISO standard source provided in the second column of Table 1 of ISO 7589:2002. 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 illumination For tungsten measurements without the camera lens, the ISO sensitometric studio tungsten illuminant given in Table 2 of ISO 7589:2002 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 SIST ISO 12232:2011
ISO 12232:2006(E) © ISO 2006 – All rights reserved
5to the tungsten ISO standard source provided in the second column of Table 2 of ISO 7589:2002. 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, as specified in ISO 554, 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(DH) or σ(DL) in Clause 6. If it is not possible to disable the camera compression, 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, for example with the DSC set to monochrome mode. 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. 6 Determination of ISO speed 6.1 General With appropriate electrical or digital gain, a DSC can provide an appropriate output signal level for a range of sensor exposure levels. The maximum exposure level is the exposure level where typical picture highlights will be clipped as a result of saturating the image sensor signal capacity or reaching the camera signal processing maximum signal level. The minimum exposure level depends on the amount of noise that can be tolerated in the image. These situations lead to two different types of speed values, saturation signal-based values and noise-based values. The ISO speed is preferably determined using a noise-based method. The saturation-based value is preferably used to indicate the camera's overexposure speed latitude. A second noise-based value is preferably used to indicate the camera's underexposure speed latitude. For some types of DSCs, such as those employing lossy compression methods, it is not possible to correctly determine the noise-based ISO speed. In such cases, the ISO speed of the camera is determined using the saturation-based measurement, and the ISO speed latitude values are not reported. In other cases, the noise-based ISO speed may be lower than the saturation-based speed, in which case the saturation based-speed is reported. SIST ISO 12232:2011
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© ISO 2006 – All rights reserved 6.2 Saturation-based speed In photographic applications where the scene illumination level can be controlled, for example in studio photography, the photographer normally prefers to use a camera exposure index which provides the best possible image quality. In this situation, a saturation signal-based rating is appropriate. This rating allows the user to set the camera exposure so that typical image highlights are just below the maximum possible (saturation) camera signal value. 6.2.1 Focal plane measurement The saturation based speed, Ssat, of an electronic still picture camera is defined as: Ssat = 78/Hsat (5) where Hsat is the minimum focal plane exposure, expressed in lux-seconds (lx⋅s), that produces the maximum valid (not clipped or bloomed) camera output signal. NOTE Equation (5) provides 1/2 “stop" of headroom (41 % additional headroom) for specular highlights above the signal level that would be obtained from a theoretical 100 % reflectance object in the scene, so that a theoretical 141 % reflectance object in the scene would produce a focal plane exposure of Hsat. Therefore, an 18 % reflectance test card in the scene would produce a focal plane exposure of 128/1 000 Hsat. Thus, the multiplicative constant 78 in Equation (5) is equal to 10 times 1 000/128, where the value 10 is the constant from Equation (1). 6.2.2 Scene luminance measurement If the focal plane exposure of the DSC
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