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The Photographic Sensitivity of Electronic Still Cameras

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The Photographic Sensitivity of Electronic Still Cameras ―117― J. Soc. Photogr. Sci. Technol. Japan, Vol.59, No.1, 1996 The Photographic Sensitivity of Electronic Still Cameras Jack HOLM* Abstract This paper presents an overview of electronic still camera photographic sensitivity (speed) determination procedures and concepts. The approach used for film speed and exposure determination is described, followed by a review of the work done in ISO TC42/ WG18 toward the development of electronic still and digital camera speed standards. The protocol developed for the determination of saturation based speed is given. The theoretical and experimental work on the signal- to- noise based speed concept is reviewed, including the subjective correlation between EIQ values, midtone log NEQ values, and the noise related subjective image quality. The effect of different pixel pitches is briefly described, and some common film EIQ values are noted in comparison to digital camera values and subjective perceptions. Preliminary protocols for noise based speed determination are provided, and the relation between noise based speed and DQE is noted. Preliminary work on color camera noise measurement is also outlined. ture it. Consequently, the optical system of the Introduction camera should be set up to deliver the optimum The art and science of photography enable amount of radiation to the sensor. With film humanity to communicate, and record and systems, the primary characteristic used to explore the universe. In some respects the determine the appropriate amount of radiation is phrase "seeing is believing" applies to the view- the film speed. Films with higher speeds are ing of photographic images. Pictures can be more sensitive and therefore capture scenes in manipulated to mislead, but most often the inten- less time. tion of the photographer is to show a perception Film speed designation procedures developed of reality. In photojournalism and the sciences over a number of decades, beginning with the in particular, the intention is frequently to show work of Hurter& Driffield and several astro- reality with greater clarity than can easily be nomical photographers in the 19th century, and seen by the human visual system. Unfortunate- culminating with the work of Jones& Condit1 in ly, the universe which can be photographically the 1940's.* Current film speed designation pro- recorded is limited. One limitation is the num- cedures are under continual review by the Inter- ber of possible locations of the camera; another national Organization for Standardization is the sensitivity of the camera. If the amount (ISO), and are relatively stable. The speed of radiation to be recorded is too small to regis- designations resulting from these procedures are ter, a picture cannot be obtained. This limita- application dependent- it is possible for one film tion applies to photographs of couples by candle- type to have different speeds for different uses. light as well as quasars at the edge of the uni- * Although the paper by Jones& Condit is considered by many verse. In addition to defining the "photographable to be the pivotal paper relating to speed and exposure deter- mination, the work presented therein is based on a broad universe," sensitivity ratings are necessary for foundation of research conducted by a number of important efficient photography. The dynamic range of a photographers and photographic scientists, and has been scene can be quite large, and the full range of a subsequently refined and expanded by a number of individ- modern film or sensor may be required to cap- uals. * Consultant 7352 East River Road , Rush, NY 14543- 9769, USA ―118― Jack HoLm J. Soc. Photogr. Sci. Technol. Japan Since speed ratings are designed to be used with square meter, f# is the camera lens effective specific exposure determination systems, they f- number, t is the exposure time in seconds, are related to but different from absolute sensi- <Ha> is the predicted arithmetic mean focal tivity. plane exposure in lux-seconds, <La> is the As electronic photography has emerged, it has measured arithmetic mean scene luminance become evident that a speed rating system simi- in candelas per square meter, and E is the lar to that used for film is needed. However the measured scene illuminance in lux.•`= signs characteristics of electronic detectors are indicate an equality which holds for a statis- different than those of film. In determining tically average•õscene, but not for all scenes. speed ratings for electronic cameras, it is neces- sary to step back and examine the philosophical Note: Averaging reflected light exposure meters, the approach used to establish film speeds. It is also most common type, measure <La>. Unfortunately, necessary for the development of electronic it is more useful to measure <Lg>, as <Hg> is more speed ratings to take considerably less than sixty stable than <Ha>, and the log of <Hg> is approximate- years. ly halfway between the minimum and maximum Fortunately, much of the overall exposure focal plane log exposures for typical scenes. This makes the use of <Hg> more appropriate for subse- determination approach used for film can be quent calculations. For statistically average applied to and expanded for electronic cameras. scenes, <Hg> can be estimated to be approximately In addition to reducing the amount of develop- 0.8<Ha>. Measurement of <Lg> can be accom- ment required, this approach is motivated by the plished through multiple spot readings, or estimated large base of photographers educated in film using incident readings. exposure determination. It is therefore highly desirable that any electronic photography speed 2. Select a reference focal plane exposure Href ratings be consistent with the film exposure which produces a critical sensitivity depen- determination approach. To this end, work has dent film characteristic. Href should be cho- been progressing toward the development of ISO sen carefully to relate to the most critical speed ratings for electronic cameras and sensors. exposure for a particular application. This work is being done largely by the members 3. Determine the multiplicative factor •¢H of the ISO Technical Committee 42, Photogra- relating Href to the geometric mean focal phy, Working Group 18, Electronic still picture plane exposure <Hg>: imaging (ISO TC42/ WG18). Film Speeds and Exposure Determination (2) The following is a brief overview of the cur- 4. Calculate the speed constant k: rent film speed and exposure determination pro- cedures (more detail can be found in other (3) sources2, 3) 5. Determine the applicable film speed using 1. Calibrate the exposure meter so that when it the equation: is set on a particular exposure index (EI), it † will indicate camera exposure time and lens A statistically average scene is the ensemble average of the luminance distributions of all scenes ever photographed. A f- number settings according to the following number of studies have been conducted to estimate the char- equation : acteristics of a statistically average scene, and the following values are common for pictorial still photography : Scene luminance ratio 160: 1 Focal plane illuminance (or exposure) ratio 80: 1 Scene geometric mean reflectance 14.4% (1) Scene arithmetic mean reflectance 18% Scene highlight reflectance 128% (specular) In equation 1,<Hg> is the predicted geomet- Scene shadow reflectance 0.81% ric mean focal plane exposure in lux- It is important to remember, however, that scenes are highly variable, and that most scenes are not statistically average. seconds,<Lg> is the measured geometric For example, the standard deviation in the log luminance mean scene luminance in candelas per range for all scenes has been estimated to be 0.38log units . The Photographic Sensitivity of Vol.59, No.1, 1996 Electronic Still Cameras ―119― Electronic Still Picture Cameras (4) In electronic photography, the situation is Note: If the film speed is determined using ISO different. The following quotes are from a standard procedures, including the choice of Href and working draft of the electronic speed standard the constant k (which implies a specific •¢H), the under development: 4 "When an image from an electronic still pic speed value obtained is an ISO speed. - 6. Set the film speed on the exposure meter as ture camera is obtained using an insufficient the exposure index and determine the appro- exposure, proper tone reproduction can gener- ally be maintained by increasing the electronic priate camera exposure settings for each scene photographed. This step can be per- gain, but the image will contain an unacceptable formed automatically by many modern amount of noise. As the exposure is increased, cameras. the gain can be decreased, and therefore the image noise can normally be reduced to an Note: The value set on the exposure meter is the acceptable level. If the exposure is increased exposure index. This value is frequently, but incor- rectly, labeled the ISO [speed], because for normal excessively, the resulting signal in bright areas exposure determination the exposure index should of the image may exceed the maximum signal be equal to the ISO speed. The ISO speed, however, level capacity of the image sensor or camera is a characteristic of the film. The exposure index signal processing. This can cause the image is an index set on an exposure meter to determine highlights to be clipped to form a uniformly camera exposure settings. In some circumstances, bright area, or to bloom into surrounding areas the optimal exposure settings will be obtained using of the image. an exposure index which is different from the ISO speed. This is particularly true for scenes which The maximum exposure level is the exposure are substantially different from the statistically aver- level where typical picture highlights will be age scene, or when unusual film processing tech- clipped as a result of saturating the image sensor niques are employed. Non- standard processing signal capacity or reaching the camera signal techniques can also result in actual film speeds which processing maximum signal level.
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