AS ISO 12232:2019 pdf download – Photography – Digital still cameras – Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index
4.2  Focal plane measurement
For DSCs where it is possible to measure the focal plane exposure directly, such as a DSC with a removable lens, the value of H m should be measured using spatially uniform and spectrally neutral illumination. The measurement should be made within a circle lying in the centre of the focal plane with a diameter of 75/100 times the shorter dimension of the image field.
4.3  Estimating focal plane exposure from scene luminance
For DSCs where it is not practical to measure the focal plane exposure, the value of H m required in Formula (1) should be estimated using Formula (2). The measurement should be made using a test card for a flat-field image (i.e. a spatially uniform field-of-view of spectrally neutral reflectance) at a distance of at least one meter, using only the portion of the field of view of the image sensor that is subtended by an angle of less than 10°. If the lens focus can be manually adjusted, it should be set to the infinity position. The derivation of Formula (2) is given in Annex B.
5.1  General
The following measurement conditions should be used as nominal conditions when determining the ISO speed ratings, SOS, and REI values of a DSC. If it is not possible or not appropriate to use these nominal operating conditions, the actual operating conditions shall be listed along with the reported values.
5.2  Illumination
The reported values shall indicate whether illumination approximating the ISO 7589 daylight or studio tungsten illuminant was used. ISO 7589 describes the procedures for determining if the illumination used in a specific speed rating determination test is an acceptable match to the daylight and studio tungsten illuminants. Also, the spectral power distribution of the illumination used should be reasonably similar to that of the ISO 7589 daylight or studio tungsten reference illuminant.
5.2.1  Daylight reference illuminant
For daylight measurements without the camera lens, the ISO sensitometric daylight illuminant given in ISO 7589:2002, Table 1, shall be used. This illuminant is defined as the product of the spectral power distribution of CIE colorimetric standard illuminant D 55 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 ISO 7589:2002, Table 1. In order to apply the ISO SDI (spectral distribution index) criterion, the spectral radiance of the light shall be measured and then multiplied by the relative spectral transmittance of the ISO standard lens, which is also described in ISO 7589, prior to multiplying by the weighted spectral sensitivities.