ISO 12232:2019

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 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

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
Foreword
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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
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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.
iv © ISO 2019 – All rights reserved

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 camera
...