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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. 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°. 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. ) 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. 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. The following table shows the correspondence between these scales: A film speed is converted from the linear scale to the logarithmic scale by this formula (plus rounding to the nearest integer): : 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 above. 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. 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 , and exposure time ( Shutter Speed ). The equation may be expressed as ratios, or, by taking the logarithm (base 2) of both sides, by addition. As a result, 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 , which in turn is proportional to the lens area by the square root. Thus, a lens set to 1.4 allows twice as much light to strike the focal plane as a lens set to 2. Therefore, each increment of the square root of two (approximately 1.4) is also a stop, so lenses are typically marked in that progression: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc. 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. Fine-grain stock, such as portrait film or those used for the intermediate stages of copying Original Camera Negative s, 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 grainy image. The image actually consists of a mosaic of developed and undeveloped areas of the emulsion, and each grain of silver halide develops in an all-or-nothing way. Thus, film is a threshold detector rather than a linear detector. If the subject has an edge between light and darkness and that edge falls on a grain, the result will be an area that is all light or all shadow. An accumulation of such areas breaks up the visible contours of the object, the effect known as graininess (or grain). Fast films are also relatively contrasty, for the same reason. That is, an area of the image will consist of bright areas and dark ones with few transitional areas of midtones. Kodak used to use a Granularity Index (GI) to characterize film grain. Alternating images of the film under test and a standard grain were shown to test subjects who indicated when they perceived a match. The standard grain samples were the index. More recently, Kodak switched to a measurement of grain using an RMS measurement. 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. 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 . Altering film speed Certain high-speed black-and-white films, such as 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. DIGITAL CAMERA SENSITIVITY The rather than Grain . The best digital cameras As Of 2005 exhibit no perceptible noise at ISO 100 sensitivity, and some produce useable results up to ISO 3200. REFERENCES
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