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 cameras. ISO 2721 uses the term nominal exposure and assumes that the nominal exposure is an arithmetic mean exposure value, which usually corresponds to the mid-tone in photographs of average scenes.
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 ISO 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.
It can also be useful to compare or record the sensitivity of a DSC, for cases where the DSC has a fixed sensitivity. The standard output sensitivity (SOS) is designed to meet this need. Likewise, it can be useful to know the EI recommended by the DSC manufacturer for a specific condition. This information is provided by the recommended exposure index (REI).
