ISO 12232:1998
(Main)Photography — Electronic still-picture cameras — Determination of ISO speed
Photography — Electronic still-picture cameras — Determination of ISO speed
Photographie — Appareils de prises de vue électroniques — Détermination de la sensibilité ISO
General Information
Relations
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 12232
First edition
1998-08-01
Photography — Electronic still-picture
cameras — Determination of ISO speed
Photographie — Appareils de prises de vue électroniques — Détermination
de la sensibilité ISO
A
Reference number
ISO 12232:1998(E)
---------------------- Page: 1 ----------------------
ISO 12232:1998(E)
Contents Page
1 Scope. 1
2 Normative references . 1
3 Definitions . 1
4 Exposure index values. 2
Test conditions.
5 3
6 Determination of ISO speed . 4
eAnnexes
A Recommended procedure for determining the noise-based
ISO speed. 10
B Scene luminance and focal plane exposure. 12
C Calculation of noise-based minimum illumination level. 13
D Bibliography . 14
© ISO 1998
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 the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
---------------------- Page: 2 ----------------------
©
ISO ISO 12232:1998(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.
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.
International Standard ISO 12232 was prepared by Technical Committee
ISO/TC 42, Photography.
Annexes A, B, C and D of this International Standard are for information
only.
Attention is drawn to the possibility that some of the elements of this
International Standard may be the subject of patent rights. ISO shall not be
held responsible for identifying any or all such patents.
iii
---------------------- Page: 3 ----------------------
©
ISO 12232:1998(E) ISO
Introduction
The ISO speed rating is an important attribute of photographic systems.
Standardization assists users and manufacturers in obtaining proper
exposures and in determining the low light capability of electronic still
cameras. The camera exposure is determined by the exposure time, the
lens aperture, the lens transmittance, the level and colour temperature of
the scene illumination, and the scene reflectance. When an image from an
electronic still-picture camera is obtained using an insufficient exposure,
proper tone reproduction can generally be maintained by increasing the
electronic 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
exposures properly. An ISO speed rating is intended to serve as such a
guide. The methods for assigning an ISO speed rating to electronic
cameras should harmonize with current photographic standards and
practice. In order to be easily understood by photographers, the ISO speed
rating for an electronic camera should directly relate to the ISO speed
rating for photographic film cameras. For example, if an electronic camera
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 for electronic cameras described in this
International Standard are intended to harmonize with ISO speed ratings
for films. However, there are differences between electronic and film
imaging systems that preclude exact equivalency in use. Cameras with
variable gain, and digital processing after the data has been captured,
allow desired tone reproduction to be achieved over a range of camera
exposures. It is therefore possible for electronic cameras to have a range
1)
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
upper and lower limits of the ISO speed latitude indicating the speed range.
___________
1) While it is also possible to use many films at a variety of camera exposures,
the behaviour of film is fundamentally different from that of electronic sensors. For
example, significant underexposure typically results in uncorrectable tone
reproduction errors in film-based photographic systems.
iv
---------------------- Page: 4 ----------------------
©
ISO ISO 12232:1998(E)
The camera measurements described in this International Standard are
performed in the digital domain, using digital analysis techniques. For
electronic cameras that include only analog outputs, the analog signal
should be digitized, so that the digital measurement can be performed. The
digitizing equipment should be characterized, so that the effects of the
digitization can be removed from the measurement results. When this is
not possible, the type of digitizing equipment used should be reported
along with the measurement results.
Since the noise performance of an image sensor may vary significantly
with exposure time and operating temperature, these operating conditions
are specified. The visibility of noise to human observers depends on the
magnitude of the noise, the apparent tone of the area containing the noise,
and the spatial frequency of the noise. The magnitude of the noise present
in an output representation depends on the noise present in the stored
image data and the contrast amplification or gain applied to the data in
producing the output. The noise visibility is different for the luminance (or
monochrome) channel and the colour (or colour difference) channels.
Therefore, this International Standard accounts for these factors in
measuring the noise-based speed and speed latitude values.
v
---------------------- Page: 5 ----------------------
©
INTERNATIONAL STANDARD ISO ISO 12232:1998(E)
Photography — Electronic still-picture cameras — Determination
of ISO speed
1 Scope
This International Standard specifies a method for assigning exposure index values, ISO speed ratings, and ISO
speed latitude ratings to electronic still-picture cameras. The standard applies to both monochrome and colour
electronic still-picture cameras.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
International Standard. At the time of publication, the editions indicated were valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain
registers of currently valid International Standards.
ISO 554:1976, Standard atmospheres for conditioning and/or testing — Specifications.
ISO 2721:1982, Photography — Cameras — Automatic controls of exposure.
ISO 7589:1984, Photography — Illuminants for sensitometry — Specifications for daylight and incandescent
tungsten.
2)
ISO 14524:— , Photography — Electronic still-picture cameras — Methods for measuring opto-electronic
conversion functions (OECFs).
ITU-R BT.709:1993, Basic parameter values for the HDTV standard for the studio and for international programme
exchange.
3 Definitions
For the purposes of this International Standard, the following definitions apply.
3.1 electronic still-picture camera: Camera incorporating an image sensor that outputs an analog or digital
signal representing a still picture, and/or records an analog or digital signal representing a still picture on a
removable media, such as a memory card or magnetic disk.
3.2 exposure index: Numerical value that is inversely proportional to the exposure provided to an image sensor to
obtain an image. Images obtained from a camera using a range of exposure index values will normally provide a
range of image quality levels.
___________
2) To be published.
1
---------------------- Page: 6 ----------------------
©
ISO
ISO 12232:1998(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; for
example, a charge-coupled device (CCD) array.
3.5 ISO speed: Numerical value calculated from the exposure provided at the focal plane of an electronic camera
to produce specified camera output signal characteristics using the methods described in this International
Standard. The ISO speed should correlate with the highest exposure index value that provides peak image quality
for normal scenes.
3.6 ISO speed latitude: Set of two numerical values calculated from the exposure provided at the focal plane of
an electronic camera to produce specified camera output signal characteristics using the methods described in this
International Standard. The ISO speed latitude should 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 signal processing: Operations performed by electronic circuits or algorithms that convert or modify the output
of an image sensor.
4 Exposure index values
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 camera, using a range of EI values, will normally provide a
range of image quality levels. The ISO speed of an electronic still-picture camera is equal to a particular exposure
index value calculated from the exposure provided at the focal plane of an electronic camera 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 an electronic still
camera 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.
4.1 Focal plane measurement
For electronic still (or other) camera 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 should be computed using equation (1), as described in ISO 2721.
10
El = . . . (1)
H
a
3)4)
where is the arithmetic mean focal plane exposure, in lux seconds .
H
a
___________
3) Note that 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 Hurter and Driffield (H and 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.
4) Note that the arithmetic mean focal plane exposure for statistically average scenes is 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 equals 2/10 times the focal plane exposure which would be obtained from
a 90 % reflectance test card in a statistically average scene.
2
---------------------- Page: 7 ----------------------
©
ISO
ISO 12232:1998(E)
4.2 Scene luminance measurement
For electronic still (or other) camera exposure meters where the arithmetic mean scene luminance is measured, the
expected value of the arithmetic mean focal plane exposure required in equation (1) can be computed using
equation (2). The derivation of equation (2) is given in annex B.
65Lt
a
H = . . . (2)
a
2
100A
where
A is the effective f-number of the lens;
L is the arithmetic mean luminance, in candelas per square metre;
a
t is the photosite integration time, in seconds.
NOTE — Note that the laboratory measurement of L can be simplified by using a full-frame uniformly illuminated diffuse-
a
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 of the lens for the focused image shall be calculated using the equation:
Effective f-number = (1 + 1/R) f-number . . . (3)
where R is the ratio of the height of the camera field of view at the focal distance to the height of the image at the
focal plane. 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
El = . . . (4)
10Lt
a
5 Test conditions
The following measurement conditions should be used as nominal conditions when determining the ISO speed
rating of an electronic still-picture camera. If it is not possible or appropriate to achieve these nominal operating
conditions, the actual operating conditions shall be listed along with the ISO speed-rating value.
5.1 Illumination
The ISO speed rating 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.
5.1.1 Daylight illumination
For daylight ISO speed measurements without the camera lens, the ISO sensitometric daylight illuminant given in
table 1 of ISO 7589:1984 shall be used. This illuminant is defined as the product of the spectral power distribution of
CIE standard colorimetric illuminant D and the spectral transmittance of the International Standard camera lens.
55
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 7589:1984. 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.
3
---------------------- Page: 8 ----------------------
©
ISO
ISO 12232:1998(E)
5.1.2 Tungsten illumination
For tungsten ISO speed measurements without the camera lens, the ISO sensitometric tungsten illuminant given in
table 2 of ISO 7589:1984 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 3050 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 table 2 of ISO 7589:1984. 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 Temperature and relative humidity
The ambient temperature during the acquisition of the test data shall be 23 °C – 2 °C, as specified in ISO 554, and
the relative humidity should be 50 % – 20 %.
5.3 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.
5.4 Infrared (IR) blocking filter
If required, an infrared (IR) blocking filter shall be used, as specified in ISO 14524.
5.5 Photosite integration time
The photosite integration time should not be longer than 1/30 s.
6 Determination of ISO speed
With appropriate electrical gain, an electronic camera 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 maximum signal level for camera
signal processing. 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 electronic still-picture cameras,
such as those employing lossy compression methods that cannot be bypassed or those with excessively high noise
levels, 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.
6.1 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 image highlights are just below the maximum possible (saturation) camera signal value.
4
---------------------- Page: 9 ----------------------
©
ISO
ISO 12232:1998(E)
6.1.1 Focal plane measurement
5)
The saturation-based speed, S , of an electronic still-picture camera is defined as :
sat
78
S = . . . (5)
sat
H
sat
where H is the minimu
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.