Editing Film speed (section)

==Digital camera ISO speed and exposure index{{anchor|Digital|ISO speed}}==
{{Unreferenced section|date=October 2016}}
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[[File:CCD Image sensor.jpg|thumb|300px|A [[Charge-coupled device|CCD]] image sensor, 2/3 inch size]]
In [[Digital camera|digital camera systems]], an arbitrary relationship between exposure and sensor data values can be achieved by setting the [[signal gain]] of the sensor. The relationship between the sensor data values and the lightness of the finished image is also arbitrary, depending on the parameters chosen for the interpretation of the sensor data into an image [[color space]] such as [[sRGB]].

For digital photo cameras ("digital still cameras"), an [[#EI|exposure index]] (EI) rating—commonly called ''ISO'' setting—is specified by the manufacturer such that the sRGB image files produced by the camera will have a lightness similar to what would be obtained with film of the same EI rating at the same exposure. The usual design is that the camera's parameters for interpreting the sensor data values into sRGB values are fixed, and a number of different EI choices are accommodated by varying the sensor's signal gain in the analog realm, prior to conversion to digital. Some camera designs provide at least some EI choices by adjusting the sensor's signal gain in the digital realm ("expanded ISO"). A few camera designs also provide EI adjustment through a choice of lightness parameters for the interpretation of sensor data values into sRGB; this variation allows different tradeoffs between the range of highlights that can be captured and the amount of noise introduced into the shadow areas of the photo.

Digital cameras have far surpassed film in terms of sensitivity to light, with [[#ISO|ISO]] equivalent speeds of up to 4,560,000, a number that is unfathomable in the realm of conventional film photography. Faster [[microprocessor]]s, as well as advances in software noise reduction techniques allow this type of processing to be executed the moment the photo is captured, allowing photographers to store images that have a higher level of refinement and would have been prohibitively time-consuming to process with earlier generations of digital camera hardware.

===The ISO (International Organization of Standards) 12232:2019 standard{{anchor|ISO 12232|REI|SOS}}===

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The ISO standard ISO&nbsp;12232:2006<ref name="iso12232"/> gave digital still camera manufacturers a choice of five different techniques for determining the exposure index rating at each sensitivity setting provided by a particular camera model. Three of the techniques in ISO&nbsp;12232:2006 were carried over from the 1998 version of the standard, while two new techniques allowing for measurement of JPEG output files were introduced from [[Camera & Imaging Products Association|CIPA]] DC-004.<ref name="CIPA_DC-004"/> Depending on the technique selected, the exposure index rating could depend on the sensor sensitivity, the sensor noise, and the appearance of the resulting image. The standard specified the measurement of light sensitivity of the entire digital camera system and not of individual components such as digital sensors, although Kodak has reported<ref name="Kodak_Sensor_ISO"/> using a variation to characterize the sensitivity of two of their sensors in 2001.

The ''Recommended Exposure Index'' (REI) technique, new in the 2006 version of the standard, allows the manufacturer to specify a camera model's [[#EI|EI]] choices arbitrarily. The choices are based solely on the manufacturer's opinion of what EI values produce well-exposed [[sRGB]] images at the various sensor sensitivity settings. This is the only technique available under the standard for output formats that are not in the sRGB color space. This is also the only technique available under the standard when [[Metering mode#Multi-zone metering|multi-zone metering]] (also called ''pattern'' metering) is used.

The ''Standard Output Sensitivity'' (SOS) technique, also new in the 2006 version of the standard, effectively specifies that the average level in the sRGB image must be 18% gray plus or minus 1/3 stop when the exposure is controlled by an automatic exposure control system calibrated per [[Light Meter|ISO&nbsp;2721]] and set to the EI with no [[exposure compensation]]. Because the output level is measured in the sRGB output from the camera, it is only applicable to sRGB {{nowrap|images—typically}} [[JPEG]]—and not to output files in [[raw image format]]. It is not applicable when multi-zone metering is used.

The CIPA DC-004 standard requires that Japanese manufacturers of digital still cameras use either the REI or SOS techniques, and DC-008<ref name="CIPA_DC-008"/> updates the [[Exchangeable image file format|Exif]] specification to differentiate between these values. Consequently, the three EI techniques carried over from ISO&nbsp;12232:1998 are not widely used in recent camera models (approximately 2007 and later). As those earlier techniques did not allow for measurement from images produced with [[lossy compression]], they cannot be used at all on cameras that produce images only in JPEG format.

The ''saturation-based'' (SAT or S<sub>sat</sub>) technique is closely related to the SOS technique, with the sRGB output level being measured at 100% white rather than 18% gray. The SOS value is effectively 0.704 times the saturation-based value.<ref name="Kerr_2007_Digital-Camera-Sensitivity"/> Because the output level is measured in the [[sRGB]] output from the camera, it is only applicable to sRGB images—typically [[Tagged Image File Format|TIFF]]—and not to output files in raw image format.{{citation needed|date=August 2019}} It is not applicable when multi-zone metering is used.

The two ''noise-based'' techniques have rarely been used for consumer digital still cameras.{{citation needed|date=August 2019}} These techniques specify the highest EI that can be used while still providing either an "excellent" picture or a "usable" picture depending on the technique chosen.{{citation needed|date=August 2019}}

An update to this standard has been published as ISO&nbsp;12232:2019, defining a wider range of ISO speeds.<ref name="ISO 12232_2019"/><ref name="Gasiorowski-Denis_2019_ISO12232"/>

===Measurements and calculations===
ISO speed ratings of a digital camera are based on the properties of the sensor and the image processing done in the camera, and are expressed in terms of the luminous exposure ''H'' (in [[lux]] [[second]]s) arriving at the sensor. For a typical camera lens with an effective [[focal length]] ''f'' that is much smaller than the distance between the camera and the photographed scene, ''H'' is given by
:<math>H=\frac{qLt}{N^2}</math>,
where ''L'' is the [[luminance]] of the scene (in [[candela]] per [[metre|m]]²), ''t'' is the exposure time (in seconds), ''N'' is the aperture f-number, and
:<math>q=\frac{\pi}{4} T\,v(\theta)\,\cos^4\theta</math>
is a factor depending on the [[transmittance]] ''T'' of the lens, the [[vignetting]] factor ''v''(''θ''), and the angle ''θ'' relative to the axis of the lens. A typical value is ''q''&nbsp;=&nbsp;0.65, based on ''θ''&nbsp;=&nbsp;10°, ''T''&nbsp;=&nbsp;0.9, and ''v''&nbsp;=&nbsp;0.98.<ref name="ISO-12232_1998"/>

====Saturation-based speed====
The ''saturation-based speed'' is defined as
:<math>S_{\mathrm{sat}}=\frac{78\;\text{lx⋅s}}{H_{\mathrm{sat}}}</math>,
where <math>H_{\mathrm{sat}}</math> is the maximum possible exposure that does not lead to a clipped or bloomed camera output. Typically, the lower limit of the saturation speed is determined by the sensor itself, but with the [[Gain (electronics)|gain]] of the amplifier between the sensor and the [[analog-to-digital converter]], the saturation speed can be increased. The factor 78 is chosen such that exposure settings based on a standard [[light meter]] and an 18-percent reflective surface will result in an image with a grey level of 18%/{{radic|2}}&nbsp;=&nbsp;12.7% of saturation. The factor {{radic|2}} indicates that there is half a stop of headroom to deal with [[specular reflection]]s that would appear brighter than a 100% reflecting diffuse white surface.<ref name="iso12232"/>

====Noise-based speed====
{{main|Signal to noise ratio (imaging)}}[[File:ISO.gif|thumb|Digital noise at 3200 ISO vs. 100 ISO]]
The ''noise-based speed'' is defined as the exposure that will lead to a given [[signal-to-noise ratio]] on individual [[pixel]]s. Two ratios are used, the 40:1 ("excellent image quality") and the 10:1 ("acceptable image quality") ratio. These ratios have been subjectively determined based on a resolution of 70&nbsp;pixels per cm (178 DPI) when viewed at 25&nbsp;cm (9.8&nbsp;inch) distance. The noise is defined as the [[standard deviation]] of a weighted average of the [[luminance]] and color of individual pixels. The noise-based speed is mostly determined by the properties of the sensor and somewhat affected by the noise in the electronic gain and AD converter.<ref name="iso12232"/>

====Standard output sensitivity (SOS){{anchor|SOS}}====
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In addition to the above speed ratings, the standard also defines the ''standard output sensitivity'' (SOS), how the exposure is related to the digital pixel values in the output image. It is defined as
:<math>S_{\mathrm{sos}} = \frac{10\;\text{lx⋅s}}{H_{\mathrm{sos}}},</math>
where <math>H_{\mathrm{sos}}</math> is the exposure that will lead to values of 118 in 8-bit pixels, which is 18 percent of the saturation value in images encoded as [[sRGB]] or with [[gamma correction|gamma]]&nbsp;=&nbsp;2.2.<ref name="iso12232"/>

====Discussion====
The standard specifies how speed ratings should be reported by the camera. If the noise-based speed (40:1) is ''higher'' than the saturation-based speed, the noise-based speed should be reported, rounded ''downwards'' to a standard value (e.g. 200, 250, 320, or 400). The rationale is that exposure according to the lower saturation-based speed would not result in a visibly better image. In addition, an exposure latitude can be specified, ranging from the saturation-based speed to the 10:1 noise-based speed. If the noise-based speed (40:1) is ''lower'' than the saturation-based speed, or undefined because of high noise, the saturation-based speed is specified, rounded upwards to a standard value, because using the noise-based speed would lead to overexposed images. The camera may also report the SOS-based speed (explicitly as being an SOS speed), rounded to the nearest standard speed rating.<ref name="iso12232"/>

For example, a camera sensor may have the following properties: <math>S_{40:1}=107</math>, <math>S_{10:1}=1688</math>, and <math>S_{\mathrm{sat}}=49</math>. According to the standard, the camera should report its sensitivity as
:''ISO&nbsp;100 (daylight)''
:''ISO speed latitude 50–1600''
:''ISO&nbsp;100 (SOS, daylight)''.
The SOS rating could be user controlled. For a different camera with a noisier sensor, the properties might be <math>S_{40:1}=40</math>, <math>S_{10:1}=800</math>, and <math>S_{\mathrm{sat}}=200</math>. In this case, the camera should report
:''ISO&nbsp;200 (daylight)'',
as well as a user-adjustable SOS value. In all cases, the camera should indicate for the white balance setting for which the speed rating applies, such as daylight or tungsten ([[incandescent light]]).<ref name="iso12232"/>

Despite these detailed standard definitions, cameras typically do not clearly indicate whether the user "ISO" setting refers to the noise-based speed, saturation-based speed, or the specified output sensitivity, or even some made-up number for marketing purposes. Because the 1998 version of ISO&nbsp;12232 did not permit measurement of camera output that had lossy compression, it was not possible to correctly apply any of those measurements to cameras that did not produce [[sRGB]] files in an uncompressed format such as [[Tagged Image File Format|TIFF]]. Following the publication of CIPA&nbsp;DC-004 in 2006, Japanese manufacturers of digital still cameras are required to specify whether a sensitivity rating is REI or SOS.{{Citation needed|date=April 2009}}

A greater SOS setting for a given sensor comes with some loss of image quality, just like with analog film. However, this loss is visible as image noise rather than [[film grain|grain]]. APS- and 35&nbsp;mm-sized digital [[image sensors]], both CMOS and CCD based, do not produce significant noise until about ''ISO&nbsp;1600''.<ref name="D200"/>