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the (709), and (248), iso (203), speed (199), film (178), for (139), exposure (111), asa (93), with (85), from (80), photography (75), din (75), was (71), camera (64), this (59), that (59), #standard (58), #system (54), sensitivity (47), weston (47), photographic (45), digital (41), edit (41), speeds (41), standards (40), films (39), archived (39), based (39), were (39), 2011 (38), light (36), negative (35), ansi (34), are (34), ph2 (33), 100 (33), retrieved (31), new (31), kodak (31), index (30), still (30), which (30), noise (29), image (28), 4512 (28), der (28), not (28), used (28), values (28), determination (27), german (26), ratings (26), white (25), american (25), general (25), original (25), international (24), been (24), also (24), 2240 (23), color (23), sensor (23), number (23), für (23), black (22), cameras (22), output (22), gost (22), rating (22), emulsion (22), systems (21), canon (21), die (21), density (21), wayback (20), plate (20), method (20), 1993 (20), meters (20), 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ed is determined from s 0 8 lx s h m displaystyle s frac 0 8 text lx s h_ mathrm m this value is then rounded to the nearest standard speed in table 1 of iso 6 1993 determining speed for color negative film is similar in concept but more complex because it involves separate curves for blue green and red the film is processed according to the film manufacturer s recommendations rather than to a specified contrast iso speed for color reversal film is determined from the middle rather than the threshold of the curve it again involves separate curves for blue green and red and the film is processed according to the film manufacturer s recommendations applying film speed edit film speed is used in the exposure equations to find the appropriate exposure parameters four variables are available to the photographer to obtain the desired effect lighting film speed f number aperture size and shutter speed exposure time the equation may be expressed as ratios or by taking the logarithm base 2 of both sides by addition using the apex system in which every increment of 1 is a doubling of exposure this increment is commonly known as a stop the effective f number is proportional to the ratio between the lens focal length and aperture diameter the diameter itself being proportional to the square root of the aperture area thus a lens set to f 1 4 allows twice as much light to strike the focal plane as a lens set to f 2 therefore each f number factor of the square root of two approximately 1 4 is also a stop so lenses are typically marked in that progression f 1 4 2 2 8 4 5 6 8 11 16 22 32 etc the iso arithmetic speed has a useful property for photographers without the equipment for taking a metered light reading correct exposure will usually be achieved for a frontlighted scene in bright sun if the aperture of the lens is set to f 16 and the shutter speed is the reciprocal of the iso film speed e g 1 100 second for 100 iso film this is known as the sunny 16 rule exposure index edit exposure index or ei refers to speed rating assigned to a particular film and shooting situation in variance to the film s actual speed it is used to compensate for equipment calibration inaccuracies or process variables or to achieve certain effects the exposure index may simply be called the speed setting as compared to the speed rating for example a photographer may rate an iso 400 film at ei 800 and then use push processing to obtain printable negatives in low light conditions the film has been exposed at ei 800 another example occurs where a camera s shutter is miscalibrated and consistently overexposes or underexposes the film similarly a light meter may be inaccurate one may adjust the ei setting accordingly in order to compensate for these defects and consistently produce correctly exposed negatives reciprocity edit upon exposure the amount of light energy that reaches the film determines the effect upon the emulsion if the brightness of the light is multiplied by a factor and the exposure of the film decreased by the same factor by varying the camera s shutter speed and aperture so that the energy received is the same the film will be developed to the same density this rule is called reciprocity the systems for determining the sensitivity for an emulsion are possible because reciprocity holds over a wide range of customary conditions in practice reciprocity works reasonably well for normal photographic films for the range of exposures between 1 1000 second to 1 2 second however this relationship breaks down outside these limits a phenomenon known as reciprocity failure 70 film sensitivity and grain edit main article film grain grainy high speed b w film negative the size of silver halide grains in the emulsion affects film sensitivity which is related to granularity because larger grains give film greater sensitivity to light fine grain film such as film designed for portraiture or copying original camera negatives is relatively insensitive or slow because it requires brighter light or a longer exposure than a fast film fast films used for photographing in low light or capturing high speed motion produce comparatively grainy images kodak has defined a print grain index pgi to characterize film grain color negative films only based on perceptual just noticeable difference of graininess in prints they also define granularity a measurement of grain using an rms measurement of density fluctuations in uniformly exposed film measured with a microdensitometer with 48 micrometre aperture 71 granularity varies with exposure underexposed film looks grainier than overexposed film marketing anomalies edit some high speed black and white films such as ilford delta 3200 and p3200 t max are marketed with film speeds in excess of their true iso speed as determined using the iso testing method according to the respective data sheets the ilford product is actually an iso 1000 film 72 while the kodak film s speed is nominally 800 to 1000 iso 50 51 the manufacturers do not indicate that the 3200 number is an iso rating on their packaging 73 kodak and fuji also marketed e6 films designed for pushing hence the p prefix such as ektachrome p800 1600 and fujichrome p1600 both with a base speed of iso 400 the dx codes on the film cartridges indicate the marketed film speed i e 3200 not the iso speed in order to automate shooting and development digital camera iso speed and exposure index edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed october 2016 learn how and when to remove this message a ccd image sensor 2 3 inch size in 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 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 equivalent speeds of up to 4 560 000 a number that is unfathomable in the realm of conventional film photography faster microprocessors 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 edit the iso standard iso 12232 2006 74 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 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 cipa dc 004 75 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 76 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 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 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 iso 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 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 77 updates the exif specification to differentiate between these values consequently the three ei techniques carried over from iso 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 sat 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 78 because the output level is measured in the srgb output from the camera it is only applicable to srgb images typically tiff and not to output files in raw image format citation needed 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 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 an update to this standard has been published as iso 12232 2019 defining a wider range of iso speeds 42 43 measurements and calculations edit 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 seconds 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 h q l t n 2 displaystyle h frac qlt n 2 where l is the luminance of the scene in candela per m ² t is the exposure time in seconds n is the aperture f number and q π 4 t v θ cos 4 θ displaystyle q frac pi 4 t v theta cos 4 theta 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 0 65 based on θ 10 t 0 9 and v 0 98 79 saturation based speed edit the saturation based speed is defined as s s a t 78 lx s h s a t displaystyle s_ mathrm sat frac 78 text lx s h_ mathrm sat where h s a t displaystyle h_ mathrm sat 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 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 2 12 7 of saturation the factor 2 indicates that there is half a stop of headroom to deal with specular reflections that would appear brighter than a 100 reflecting diffuse white surface 74 noise based speed edit main article signal to noise ratio imaging 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 pixels 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 pixels per cm 178 dpi when viewed at 25 cm 9 8 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 74 standard output sensitivity sos edit 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 s s o s 10 lx s h s o s displaystyle s_ mathrm sos frac 10 text lx s h_ mathrm sos where h s o s displaystyle h_ mathrm sos 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 2 2 74 discussion edit 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 74 for example a camera sensor may have the following properties s 40 1 107 displaystyle s_ 40 1 107 s 10 1 1688 displaystyle s_ 10 1 1688 and s s a t 49 displaystyle s_ mathrm sat 49 accor...