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 (Digital Still Camera) 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.

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,



where Ha is the arithmetic mean focal plane exposure, expressed in lux-seconds (lx•s).

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.

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.

For DSC 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). 




where
A 		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.

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, Nfeff, of the lens for the focused image shall be calculated using Equation (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. 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).





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.

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.

For daylight measurements without the camera lens, the ISO sensitometric daylight illuminant given in Table 2 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 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.

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 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 7589:2002. 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.

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) %.

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. If required, an infrared (IR) blocking filter shall be used as specified in ISO 14524.

The photosite integration time should not be longer than 1/30 s. 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 the following clause. If it is not possible to disable the camera compression, the noise-based values cannot be properly determined and shall not be reported.

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.

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.

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.

The saturation based speed, Ssat, of an electronic still picture camera is defined as




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.

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 128/1000 Hsat. Thus, the multiplicative constant 78 in Equation (5) is equal to 10 times 1000/128, where the value 10 is the constant from Equation (1).

If the focal plane exposure of the DSC cannot be measured directly, it shall be computed from the scene luminance using Equation (2).

In many photographic applications, it is desirable to use the highest exposure index (i.e., the lowest exposure) possible, in order to maximize the depth of field, minimize the exposure time, and offer the maximum acceptable latitude for exposure of image highlights. An exposure index that provides an appropriately low noise image for a typical DSC is called a “noise-based speed.” The value is based on an objective correlation to subjective judgements of the acceptability of various noise levels in exposure series images. Two different noise-based speeds are determined, one (Snoise40) that provides the “first excellent” image and a second (Snoise10) that provides the “first acceptable” image. The recommended procedure for determining these noise-based speeds is given in Annex A.

The two noise-based speeds of a DSC, Snoise40 and Snoise10, shall be determined from the focal plane exposure required to produce specific image incremental signal-to-noise (S/N) ratio values, measured using linearized output signals from the DSC, using the following equations97






where HS/N40 is the exposure that provides DSC output signals which, when linearized, satisfy the equation




and HS/N10 is the exposure that provides DSC output signals which, when linearized, satisfy the equation




where H is the input photometric exposure, in lux-seconds, needed to produce the linearized luminance signal level D, and σ(D) is the standard deviation of the linearized monochrome output level values at the linearized signal level D (for monochrome cameras) or standard deviation of the linearized, weighted colour DSC output values (for colour cameras, as provided in 6.2.3), taken from a 64 by 64 pixel area.98

The DSC output signals shall be linearized in accordance with ISO 14524, and the linearized values shall be filtered using the filter provided in Annex D prior to determining σ(D). If a DSC is too noisy to meet the HS/N40 criterion, the saturation based value shall be reported as the ISO speed of the DSC.

If the focal plane exposure of the DSC cannot be measured directly, it shall be computed from the scene luminance using Equation (2).

The noise of the luminance and colour difference signals shall be determined from CRT display output-referred RGB colour signals based on the ITU-R BT.709 RGB primaries and white point, such as the sRGB and sYCC signals defined in IEC 61966-2-1, which are used as output signals in many DSCs.

For colour cameras using a single exposure process, σ(D) shall be determined using the linearized DSC output signals. If the DSC provides CRT display output-referred RGB colour signals based on the ITU-R BT.709 primaries and white point, these signals shall be converted to linearized RGB signals in accordance with ISO 14524. If the DSC encodes these RGB signals as Y, Cr, Cb output signals, the signals shall be decoded to provide RGB output signals using the inverse of the matrix used to encode the signals. The decoded RGB output signals shall then be converted to linearized RGB signals in accordance with ISO 14524.

If the DSC colour output signals are not CRT display output-referred RGB colour signals based on the ITU-R BT.709 primaries and white point, they shall be converted to the required signals, using an appropriate colour space conversion and rendering process, if necessary, prior to performing the noise analysis.

The linearized luminance signal shall be formed from the linearized RGB signals using the equation



The standard deviation of the camera noise, σ(D), shall be computed using the following equation



If the DSC has quantization steps which are similar in magnitude to, or larger than, the measured standard deviation, quantization effects may result in the measured standard deviation being incorrect. This type of error may be corrected to some extent by repeated measurements on different image files, but if the actual standard deviation is small, even repeated measurements may result in the value determined being too low. To compensate for this effect, the value of σ(H) or σ(L) used in Equations (10) and (11) shall be not less than 1/2.

The value of 1/2 is greater than the standard deviation of noise from uniform quantization, which equals the square-root of 1/12. The value of 1/2 has been chosen because if the measured standard deviation is below this value, the measured values are significantly influenced by quantization effects and are no longer meaningful.

The ISO speed of a DSC shall be denoted “ISO xxx D” (or alternatively “ISO xxx”) for daylight illumination and “ISO xxx T” for tungsten illumination. If Snoise40 is higher than Ssat, the reported number “xxx” shall be the value from the third column of Table 2 from the same row as the Snoise40 value (in the second column of Table 2) determined in 6.2. The ISO speed latitude shall be denoted “ISO Speed Latitude yyy - zzz D” (or alternatively “ISO Speed Latitude yyy – zzz:”) for daylight illumination and “ISO Speed Latitude yyy - zzz T” for tungsten illumination. The reported number “yyy” shall be the value from the third column of Table 2 from the same row as the Ssat value in the first column of Table 2. The reported number “zzz” shall be the value from the third column of Table 2 from the same row as the Snoise10.

Some DSCs form a colour image using a monochrome image sensor and a colour filter wheel to provide colour sequential image records. These cameras may use different photosite integration times, or different lens apertures, for the different colour sequential exposures. For such cameras, the ISO speed and ISO speed latitude of each colour should be measured and reported separately for each colour.

The ISO speed ratings reported in image file headers shall conform to the reporting requirements outlined above. Since the user controls on DSC adjust the exposure index used to capture each image rather than the ISO speed of the DSC, the user controls should be labelled as “exposure index” or “exposure setting” controls, rather than as “ISO speed” controls.

The “standard output sensitivity” (SOS) is the exposure index value (ISOS) for a DSC that provides a still image with a specified digital output signal value under specified test conditions. Unique SOS values can only be determined for DSC operational modes where the electronic or digital gain is fixed, and, therefore, the DSC SOS shall be reported as “variable” for DSC operational modes where the electronic or digital gain is variable. However, in this case, the ISOS corresponding to the gain or digital processing used to create a particular image file may be reported in the file header, and the range of ISOS values a particular DSC can produce may be reported.

The SOS (ISOS) shall be computed using the following equation




where HSOS is the exposure required to produce the specified standard level digital signal output equal to




where OMAX is the maximum output value of the digital system. For 8-bit systems, the reference level shall be 118.

If a camera OECF chart is used to determine SOS values, the illumination level should be 2000 lx at the chart surface for reflection test charts, and 637 cd/m2 for the most transparent portions of transparency charts.

If the DSC includes a user-controlled sensitivity setting, it shall be set to one or more specific levels, which shall be reported along with the measurement results.

The value calculated using Equation (12) shall be rounded off using Table 2 and reported as the “Standard Output Sensitivity (ISOS). A “D” or descriptive term such as “Daylight” can be used to designate daylight illumination but is not required. A “T” or descriptive term such as “tungsten” shall be used to designate tungsten illumination. An example of acceptable reporting is as follows:




It is possible that the ISOS value changes as a function of the f/number of the lens, for example, due to the structure of a microlens overlay on the image sensor. In such cases, the f/number used for the measurement shall be reported along with the ISOS value.

The DSC recommended exposure index (IREI) is a numerical value that is recommended by the DSC provider as a reference. The IREI can be used to provide appropriate settings for photographic accessories, such as exposure meters and strobe lights.

When the DSC includes a manual exposure mode, or includes an exposure mode using a simple automatic exposure function, then the IREI value is useful. However, when the DSC includes only a sophisticated automatic exposure function, which adjusts the exposure level based on the subject pattern or the absolute luminance range in the scene, the IREI value is not useful and should not be reported.

The DSC recommended exposure index shall be computed using the following equation:




where Hm is the arithmetic mean focal plane exposure, expressed in lux-seconds, recommended by the DSC provider.

If the recommended exposure index varies as a function of camera mode settings or environmental conditions, these factors shall be reported. Unless otherwise indicated, the default camera mode settings shall be used.

The Hm should be reported for both daylight and tungsten illumination.

The value calculated using Equation (14) shall be rounded off using Table 2 and reported as the “recommended exposure index” (IREI). A “D” or descriptive term such as “Daylight” can be used to designate daylight illumination but is not required. A “T” or descriptive term such as “tungsten” shall be used to designate tungsten illumination. An example of acceptable reporting is as follows:




Ssat
Snoise
ISOS & IREI
Value
8 < Ssat < 10
10 < Snoise < 12
8,909 < x < 11,22
10
10 < Ssat < 12
12 < Snoise < 16
11,22 < x < 14,14
12
12 < Ssat < 16
16 < Snoise < 20
14,14 < x < 17,82
16
16 < Ssat < 20
20 < Snoise < 25
17,82 < x < 22,45
20
20 < Ssat < 25
25 < Snoise < 32
22,45 < x < 28,28
25
25 < Ssat < 32
32 < Snoise < 40
28,28 < x < 35,64
32
32 < Ssat < 40
40 < Snoise < 50
35,64 < x < 44,90
40
40 < Ssat < 50
50 < Snoise < 64
44,90 < x < 56,57
50
50 < Ssat < 64
64 < Snoise < 80
56,57 < x < 71,27
64
64 < Ssat < 80
80 < Snoise < 100
71,27 < x < 89,09
80
80 < Ssat < 100
100 < Snoise < 125
89,09 < x < 112,2
100
100 < Ssat < 125
125 < Snoise < 160
112,2 < x < 141,4
125
125 < Ssat < 160
160 < Snoise < 200
141,4 < x < 178,2
160
160 < Ssat < 200
200 < Snoise < 250
178,2 < x < 224,5
200
200 < Ssat < 250
250 < Snoise < 320
224,5 < x < 282,8
250
250 < Ssat < 320
320 < Snoise < 400
282,8 < x < 356,4
320
320 < Ssat < 400
400 < Snoise < 500
356,4 < x < 449,0
400
400 < Ssat < 500
500 < Snoise < 640
449,0 < x < 565,7
500
500 < Ssat < 640
640 < Snoise < 800
565,7 < x < 712,7
640
640 < Ssat < 800
800 < Snoise < 1000
712,7 < x < 890,9
800
800 < Ssat < 1000
1000 < Snoise < 1250
8,909 < x < 1 122
1 000
1000 < Ssat < 1250
1250 < Snoise < 1600
1 122 < x < 1 414
1 250
1250 < Ssat < 1600
1600 < Snoise < 2000
1 414 < x < 1 782
1 600
1600 < Ssat < 2000
2000 < Snoise < 2500
1 782 < x < 2 245
2 000
2000 < Ssat < 2500
2500 < Snoise < 3200
2 245 < x < 2 828
2 500
2500 < Ssat < 3200
3200 < Snoise < 4000
2 828 < x < 3 564
3 200
3200 < Ssat < 4000
4000 < Snoise < 5000
3 564 < x < 4 490
4 000
4000 < Ssat < 5000
5000 < Snoise < 6400
4 490 < x < 5 657
5 000
5000 < Ssat < 6400
6400 < Snoise < 8000
5 657 < x < 7 127
6 400
6400 < Ssat < 8000
8000 < Snoise < 10000
7 127 < x < 8 909
8 000