Film speed is the measure of a photographic film's sensitivity to light. This article is mainly concerned with Still photography film For Motion picture film please see Film stock. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Film with lower sensitivity (lower ISO/ASA speed) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO/ASA speed) can shoot the same scene with a shorter exposure and is called a fast film. In Photography, exposure is the total amount of Light allowed to fall on the photographic medium ( Photographic film or Image sensor) during the

In the first approximation the amount of light energy which reaches the film determines the effect on the emulsion, so that if the brightness of the light is multiplied by a factor and the exposure of the film decreased by the same factor so that the energy received is the same, the film will be exposed to the same density; this rule is called reciprocity, and the concept of a unique speed for an emulsion is possible because reciprocity holds. In Photography and Holography, reciprocity refers to the inverse relationship between the intensity and duration of light that determines exposure of light-sensitive In practice this holds reasonably well for normal photographic films for the range of exposures usually used, say 1/1000 sec to 1 sec, but longer exposures, different for different films, are required outside these limits, a phenomenon known as reciprocity failure. In Photography and Holography, reciprocity refers to the inverse relationship between the intensity and duration of light that determines exposure of light-sensitive

Contents 1 Technical information 1.1 ISO film speed scales 1.2 Determining film speed 1.3 Applying film speed 1.4 Exposure index 1.5 Film grain 1.6 Improvements in film 1.7 Use of grain 1.8 Altering film speed 2 Digital camera ISO speed and exposure index 3 References 4 See also 5 External links

Technical information

ISO film speed scales

International standard ISO 5800:1987 from the International Organization for Standardization (ISO) defines both an arithmetic scale and a logarithmic scale for measuring color-negative film speed. Related standards ISO 6:1993 and ISO 2240:2003 define scales for speeds of black-and-white negative film and color reversal film.

In the ISO arithmetic scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed. In the ISO logarithmic scale, which corresponds to the older DIN scale, doubling the speed of a film implies adding 3° to the numeric value that designates the film speed. Deutsches Institut für Normung eV ( DIN; in English, the German Institute for Standardization) is the German national organization for For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.

Commonly, the logarithmic speed is omitted, and only the arithmetic speed is given (e. g. , “ISO 100”). In such cases, the quoted “ISO” speed is essentially the same as the older “ASA” speed.

GOST (Russian: ГОСТ) is a pre-1987 arithmetic standard used in the former Eastern Bloc. GOST refers to a set of technical standards maintained by the Euro-Asian Council for Standardization Metrology and Certification (EASC a regional Standards organization During the Cold War, the term Communist Bloc (or Soviet Bloc) was used to refer to the Soviet Union and countries it either controlled or that were It was almost, but not quite identical to the ASA standard. After 1987 the GOST scale was aligned to the ISO scale. GOST markings are only found on pre-1987 photographic equipment (film, cameras, lightmeters, etc. A light meter is a device used to measure the amount of Light. ) of Eastern Bloc manufacture.

The most common ISO film speeds are 25/15°, 50/18°, 100/21°, 200/24°, 400/27°, 800/30°, 1600/33°, and 3200/36°. Consumer film speeds are generally between 100/21° and 800/30°, inclusive.

Conversion from the logarithmic speed S° to the arithmetic speed S is given by^[1]

and rounding to the nearest standard arithmetic speed in the table below. By simple rearrangement, conversion from arithmetic speed to logarithmic speed is given by

and rounding to the nearest integer.

The following table shows the correspondence among the various speed scales:

ISO arithmetic scale (old ASA scale)	ISO log scale (old DIN scale)	GOST (Soviet pre-1987)	Example of film stock with this nominal speed
6	9°		original Kodachrome
8	10°
10	11°		Kodachrome 8 mm film
12	12°	11	Gevacolor 8 mm reversal film
16	13°	11	Agfacolor 8 mm reversal film
20	14°	16	Adox CMS 20
25	15°	22	old Agfacolor, Kodachrome 25
32	16°	22	Kodak Panatomic-X
40	17°	32	Kodachrome 40 (movie)
50	18°	45	Fuji RVP (Velvia)
64	19°	45	Kodachrome 64, Ektachrome-X
80	20°	65	Ilford Commercial Ortho
100	21°	90	Kodacolor Gold, Kodak T-Max (TMX)
125	22°	90	Ilford FP4, Kodak Plus-X Pan
160	23°	130	Fuji Pro 160C/S, Kodak High-Speed Ektachrome
200	24°	180	Fujicolor Superia 200
250	25°	180
320	26°	250	Kodak Tri-X Pan Professional (TXP)
400	27°	350	Kodak T-Max (TMY), Tri-X 400, Ilford HP5
500	28°	350
640	29°	560	Polaroid 600
800	30°	700	Fuji Pro 800Z
1000	31°	700	Ilford Delta 3200 (see text below)
1250	32°
1600	33°	1400–1440	Fujicolor 1600
2000	34°
2500	35°
3200	36°	2800–2880	Kodak P3200 TMAX
4000	37°
5000	38°
6400	39°

Determining film speed

Film speed is found from a plot of optical density vs. In Optics, density is a unitless measure of the Transmittance of an optical element for a given length at a given Wavelength λ: log of exposure for the film, known as the D–log H curve or Hurter–Driffield curve. Sensitometry is the scientific study of light-sensitive materials especially Photographic film. There typically are five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. For black and white negative film, the “speed point” m is the point on the curve where density exceeds the base + fog density by 0. Black-and-white is a number of Monochrome forms in Visual arts. In photography a negative may refer to three different things although they are all related 1 when the negative is developed so that a point n where the log of exposure is 1. 3 units greater than the exposure at point m has a density 0. 8 greater than the density at point m. The exposure H_m, in lux-s, is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed then is

.

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. In Photography, a reversal film is a still positive image created on a transparent base using photochemical means

Applying film speed

Film speed is used in the exposure equations to find the appropriate exposure parameters. A light meter is a device used to measure the amount of Light. Four variables are available to the photographer to obtain the desired effect: lighting, film speed, f-number (aperture size), and shutter speed (exposure time). Lighting includes both artificial Light sources such as lamps and natural illumination of interiors from Daylight. In Photography, shutter speed is the length of time a shutter is 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, known as a "stop". The effective f-number is proportional to the ratio between the lens focal length and aperture diameter, which is proportional to the square root of the aperture area. The focal length of an optical system is a measure of how strongly it converges (focuses or diverges (diffuses Light. 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.

Exposure index

Exposure index, or EI, refers to speed rating assigned to a particular film and shooting situation, and used in the exposure meter or equation, to compensate for equipment calibration inaccuracies or process variables, or to achieve certain effects. Exposure index may or may not be the same as manufacturer's film speed rating for that particular film.

The exposure index is sometimes called the speed setting, as opposed to the speed rating.

For example, a photographer may choose to rate a 400 ISO speed film at 800 and then use push processing in order to get printable negatives from low-light conditions. Push processing is a term from Photography, referring to a development technique that increases the speed of the film being processed In this case it is said that film has been shot at EI 800.

Another example of a situation when exposure index would differ from film manufacturer's rating is when a camera shutter is known to be miscalibrated and consistently overexposes or underexposes the film; similarly, a light meter can be known to understate or overstate lighting conditions. A light meter is a device used to measure the amount of Light. In such cases one could adjust EI rating accordingly in order to compensate for these effects and consistently produce correctly exposed negatives.

Film grain

Main article: Film grain

Film speed is roughly related to granularity, the size of the grains of silver halide in the emulsion, since larger grains give film a greater sensitivity to light. Film grain or granularity is the random optical texture of processed Photographic film due to the presence of small grains of a metallic silver developed from Film grain or granularity is the random optical texture of processed Photographic film due to the presence of small grains of a metallic silver developed from A silver halide is one of the compounds formed between Silver and one of the Halogens &mdash Silver bromide (AgBr chloride An emulsion ( IPA: /ɪˈmʌlʃən/ is a mixture of two Immiscible (unblendable liquids Fine-grain stock, such as portrait film or those used for the intermediate stages of copying original camera negatives, is "slow", meaning that the amount of light used to expose it must be high or the shutter must be open longer. The original camera negative (OCN is the film in a Motion picture camera which captures the original image Fast films, used for shooting in poor light or for shooting fast motion, produce a grainier image. Each grain of silver halide develops in an all-or-nothing way into dark silver or nothing. Thus, each grain is a threshold detector; in aggregate, their effect can be thought of as a noisy nonlinear analog light detector.

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. Eastman Kodak Company ( is an American multinational Public company which produces imaging and photographic materials and equipment In Psychophysics, a just noticeable difference, customarily abbreviated with lowercase letters as jnd, is the smallest difference in a specified modality of sensory 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. ^[2] Granularity varies with exposure — underexposed film looks grainier than overexposed film.

Improvements in film

In the early 1980s, there were some radical improvements in film stock. It became possible to shoot color film in very low light and produce a fine-grained image with a good range of midtones.

Use of grain

In advertising, music videos, and some drama, mismatches of grain, color cast, and so forth between shots are often deliberate and added in post-production. See also Filmmaking Post-production occurs in the making of motion pictures, television programs, Videos audio recordings

Altering film speed

Certain high-speed black-and-white films, such as Ilford Delta 3200 and Kodak T-Max P3200 (TMZ), are marketed with higher speeds on the box than their true ISO speed (determined using the ISO testing methodology). Delta is a tabular-grain Black-and-white film from Ilford Photo. Eastman Kodak Company ( is an American multinational Public company which produces imaging and photographic materials and equipment For example, the Ilford product is actually an ISO 1000 film, according to its data sheet (PDF). The manufacturers are careful not to refer to the 3200 number as an ISO speed on the packaging. These films can be successfully exposed at EI 3200 (or any of several other speeds) through the use of push processing. Push processing is a term from Photography, referring to a development technique that increases the speed of the film being processed The most sensitive sensor common in commercial photography may be the Silicon Intensified Target vidicon, at ASA 200,000, used in TV cameras. In older Video cameras before the mid to late 1980s a video camera tube or pickup tube was used instead of a Charge-coupled device (CCD

Digital camera ISO speed and exposure index

In digital camera systems, an arbitrary relationship between exposure and finished image lightness can be achieved by setting the system's signal gain. In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a This gain is not exactly the same as a sensitivity, which is why the relationship to an ISO speed is more complicated. On a camera, however, setting an ISO speed and setting the exposure accordingly, whether automatically or manually with the help of an exposure meter, should result in a photo of appropriate lightness just as with film cameras. A light meter is a device used to measure the amount of Light.

For digital photo cameras ("digital still cameras"), the ISO standard 12232:2006^[3] specifies several definitions of the speed rating depending on the sensor sensitivity, the sensor noise, and the appearance of the resulting image. The digital ISO speed ratings are related to the conventional film-speed ratings in how a standard 18 percent reflective surface would appear in an image under given lighting conditions.

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. LUX is the principal centre for the promotion and distribution of experimental Film and Video works in the UK. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units 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

where L is the luminance of the scene (in candela per m², t is the exposure time (in seconds), N is the aperture f-number, and

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. The focal length of an optical system is a measure of how strongly it converges (focuses or diverges (diffuses Light. Luminance is a photometric measure of the density of Luminous intensity in a given direction The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International In Optics and Spectroscopy, transmittance is the fraction of incident light at a specified Wavelength that passes through a sample In Photography and Optics, vignetting is a reduction of an image's brightness or saturation at the Periphery compared to the image center A typical value is q = 0. 65, based on θ = 10°, T = 0. 9, and v = 0. 98.

The saturation-based speed is defined as

where H_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 A/D-converter, the saturation speed can be increased. In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a An analog-to-digital converter (abbreviated ADC, A/D or A to D) is an electronic integrated circuit which converts continuous signals to 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. A light meter is a device used to measure the amount of Light. 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 white surface. Specular reflection is the perfect Mirror -like reflection of light (or sometimes other kinds of Wave) from a surface in which light from a single incoming

The noise-based speed is defined as the exposure that will lead to a given signal-to-noise ratio on individual pixels. Signal-to-noise ratio (often abbreviated SNR or S/N) is an Electrical engineering concept also used in other fields (such as scientific Measurements In Digital imaging, a pixel ( pict ure el ement is the smallest piece of information in an image 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 (180 DPI) when viewed at 25 cm (10 inch) distance. The signal-to-noise ratio is defined as the standard deviation of a weighted average of the luminance (overall brightness) and color of individual pixels. In Probability and Statistics, the standard deviation is a measure of the dispersion of a collection of values Luminance is a photometric measure of the density of Luminous intensity in a given direction 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.

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

where H_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. sRGB is a standard RGB (Red Green Blue color space created cooperatively by HP and Microsoft for use on monitors printers and the Internet Gamma correction, gamma nonlinearity, gamma encoding, or often simply gamma, is the name of a nonlinear operation used to code and decode luminance 2.

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.

For example, a camera sensor may have the following properties: S_40:1 = 107, S_10:1 = 1688, and S_sat = 49. According to the standard, the camera should report its sensitivity as

ISO 100 (daylight)

ISO speed latitude 50–1600

ISO 100 (SOS, daylight).

The SOS rating could be user controlled. For a different camera with a noisier sensor, the properties might be S_40:1 = 40, S_10:1 = 800, and S_sat = 200. In this case, the camera should report

ISO 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). The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general

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.

As should be clear from the above, 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 grain. Film grain or granularity is the random optical texture of processed Photographic film due to the presence of small grains of a metallic silver developed from The best digital cameras as of 2008 exhibit no perceptible noise at ISO 200 sensitivity, and some produce usable results up to ISO 25,600. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common

References

• Leslie Stroebel, John Compton, Ira Current, and Richard Zakia. Basic Photographic Materials and Processes, second edition. Boston: Focal Press, 2000. ISBN 0-240-80405-8.

See also

• APEX system
• Lens speed

External links

• The official ISO 6:1993 (black-and-white negative films; paid download). Lens speed refers to the maximum Aperture diameter or minimum F-number, of a Photographic lens.
• The official ISO 2240:2003 (color reversal films; paid download).
• The official ISO 5800:1987 (color negative films; paid download).
• The official ISO 12232:2006 (digital still cameras; paid download).
• What is the meaning of ISO for digital cameras? Digital Photography FAQ

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