Film speed is the measure of a photographic film's sensitivity to light. Stock with lower sensitivity (lower ISO speed rating) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO speed rating) can shoot the same scene with a shorter exposure and is called a fast film. This article or section does not cite its references or sources. ... For other uses, see Light (disambiguation). ... A photograph with an exposure time of 25 seconds A photograph of a night-time sky with an exposure time of 8 seconds In photography, exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a...

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 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, reciprocity refers to the relationship between different choices of aperture and shutter speed that result in identical exposure. ... In photography, reciprocity refers to the relationship between different choices of aperture and shutter speed that result in identical exposure. ...

Technical information

ISO film speed scales

The standard known as ISO 5800:1987 from the International Organization for Standardization (ISO) defines both a linear scale and a logarithmic scale for measuring film speed. “ISO” redirects here. ...

In the ISO linear 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. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°. The American National Standards Institute (ANSI) is a private, non-profit United States. ... Look up din in Wiktionary, the free dictionary. ...

Commonly, the logarithmic (DIN) component is omitted from film speed ratings, and only the linear component is given (e.g. "ISO 100"). In such cases, the quoted "ISO" rating is in effect synonymous with the older ASA standard.

GOST (Russian: ГОСТ) is a pre-1987 linear standard used in the former Eastern Bloc. 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.) of Eastern Bloc manufacture. It has been suggested that this article or section be merged with Gosstandart. ... A map of the Eastern Bloc 1948-1989. ...

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

A film speed is converted from the linear scale to the logarithmic scale by this formula (plus rounding to the nearest integer):^{[citation needed]}

Conversion from the logarithmic scale to the linear scale is analogous, except that results must be rounded to the conventional values of the linear scale listed in the table below.

The following table shows the correspondence between these scales:

It has been suggested that this article or section be merged with Gosstandart. ... Kodachrome is the trademarked name of a brand of color reversal film sold by Eastman Kodak. ... In still photography, Kodaks Kodacolor brand has been associated with various color negative films (i. ... Tri-X is a black and white photographic film from Kodak and probably the oldest film still marketed today. ...

Determining film speed

Film speed is found by referencing the Hurter–Driffield curve, or D–logE curve, for the film. This is a plot of optical density vs. log of exposure (lux-s). There are typically five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. Following the curve to the point where density exceeds the base + fog by 0.1, find the corresponding exposure. Dividing 0.8 by that exposure yields the linear ISO speed rating. Sensitometry is the scientific study of light-sensitive materials, especially photographic film. ...

Applying film speed

Film speed is used in the exposure equation 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, known as a "stop". The 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. 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. Not to be confused with lightning. ... A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ... Shutter speed can have a dramatic impact on the appearance of moving objects. ... APEX stands for Additive System of Photographic Exposure. ... This article is about focal length related to lenses and systems of lenses. ... a big (1) and a small (2) aperture For other uses, see Aperture (disambiguation). ... A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ...

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. In this case it is said that film has been shot at EI 800. Push processing is a term from photography, referring to a film development technique that increases the speed of the film being processed. ...

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. In such cases one could adjust EI rating accordingly in order to compensate for these effects and consistently produce correctly exposed negatives. A light meter is a device used to measure the amount of light. ...

Film grain

Main article: Film grain

Grainy high speed B/W film negative

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. 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. 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. 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 silver halide that have received enough photons. ... Image File history File links Korn_HP4_ISO1600. ... Image File history File links Korn_HP4_ISO1600. ... 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 silver halide that have received enough photons. ... A silver halide is one of the compounds formed between silver and one of the halogens, usually silver bromide (AgBr), silver chloride (AgCl) and silver iodide (AgI). ... A. Two immisicible liquids, not emulsified; B. An emulsion of Phase B dispersed in Phase A; C. The unstable emulsion progressively separates; D. The surfactant (purple outline) positions itself on the interfaces between Phase A and Phase B, stabilizing the emulsion An emulsion is a mixture of two immiscible (unblendable... The original camera negative is the film in a motion picture camera that captures the original image. ...

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.^[1] Granularity varies with exposure — underexposed film looks grainier than overexposed film. Eastman Kodak Company (NYSE: EK) is an American multinational public company which produces 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 input that is detectable by a human being or other animal. ...

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. Post production is the general term for the last stage of film production in which photographed scenes (also called footage) are put together into a complete film. ...

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). For example, the Ilford product is actually an ISO 1000 film, according to its data sheet. 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. The most sensitive sensor common in commercial photography may be the Silicon Intensified Target vidicon, at ASA 200,000, used in TV cameras. Delta is a tabular-grain black-and-white film from Ilford Photo. ... Eastman Kodak Company (NYSE: EK) is a large multinational public company producing photographic equipment. ... Push processing is a term from photography, referring to a film development technique that increases the speed of the film being processed. ... In older video cameras, before the 1990s, a video camera tube or pickup tube was used instead of a charge-coupled device (CCD). ...

Digital camera ISO speed and exposure index

For digital photo cameras ("digital still cameras"), the ISO standard 12232:2006^[2] 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. 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 The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... This article is about the unit of time. ... This article is about focal length related to lenses and systems of lenses. ...

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 Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... Photopic (black) and scotopic [1] (green) luminosity functions. ... This article is about the unit of length. ...

is a prefactor 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. In the fields of optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. ... An example of vignetting in a photograph Vignetting is a common feature of photographs produced by toy cameras such as this shot taken with a Holga In photography and optics, vignetting is a reduction in image brightness in the image periphery compared to the image center. ...

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. 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 white surface. In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ... 4-channel stereo multiplexed analog-to-digital converter WM8775SEDS made by Wolfson Microelectronics placed on X-Fi Fatal1ty Pro sound card An analog-to-digital converter (abbreviated ADC, A/D or A to D) is an electronic integrated circuit, which converts continuous signals to discrete digital numbers. ... A light meter is a device used to measure the amount of light. ... Diagram of specular reflection Specular reflection is the perfect, mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected onto a single outgoing direction. ...

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 (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 (overal brightness) 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 convertor. Signal-to-noise ratio (often abbreviated SNR or S/N) is an electrical engineering concept defined as the ratio of a signal power to the noise power corrupting the signal. ... This article is about the picture element. ... In probability and statistics, the standard deviation of a probability distribution, random variable, or population or multiset of values is a measure of the spread of its values. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ...

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=2.2. The title given to this article is incorrect due to technical limitations. ... Example of CRT gamma correction Plot of the sRGB standard gamma-expansion nonlinearity (red), and its local gamma value, slope in log–log space (blue). ...

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 an 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). An incandescent light bulb and its glowing filament. ...

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. The best digital cameras as of 2006 exhibit no perceptible noise at ISO 200 sensitivity, and some produce usable results up to ISO 3200. 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 silver halide that have received enough photons. ... 2006 is a common year starting on Sunday of the Gregorian calendar. ...

Images of a flower taken at ISO 100 and ISO 1600 on a Canon 400D digital camera. Both images were shot under similar lighting conditions, varying only the ISO setting and shutter speed.

ISO 100, f/5.6, 1/350 s (click on image for larger version)

ISO 1600, f/5.6, 1/4000 s (click on image for larger version)

Comparison of both images. This is a crop of a small section of each image displayed at 100%. The top portion was shot at 100 ISO, the bottom portion at 1600 ISO. The EOS 400D, called Digital Rebel XTi in North America and EOS Kiss Digital X in Japan,[2] is an entry-level digital single-lens reflex camera introduced by Canon August 24, 2006. ... Image File history File links Metadata Size of this preview: 400 × 600 pixelsFull resolution (2592 × 3888 pixel, file size: 2. ... Image File history File links Metadata Size of this preview: 400 × 600 pixelsFull resolution (2592 × 3888 pixel, file size: 4. ... Image File history File links Metadata No higher resolution available. ...

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

APEX stands for Additive System of Photographic Exposure. ... It has been suggested that Fast lens be merged into this article or section. ...

External links

• The official ISO 5800:1987 (non-free content).
• The official ISO 12232:2006 (non-free content).
• What is the meaning of ISO for digital cameras? Digital Photography FAQ
• understanding sensitivity By Gloria Putnam