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ISO film speed is a standardized rating of how sensitive a photographic film system is to light. A higher number means the film responds faster, so—if other exposure settings stay the same—it can be exposed with less light. ISO is a sensitivity rating, not a shutter speed or an exposure time.
What does ISO film speed measure?
ISO film speed expresses a film’s light sensitivity using a standardized rating method. ISO 6:1993 specifies a sensitometric method for determining and expressing the speed of photographic negative materials. Its scope is specifically black-and-white pictorial still-camera negative film and process systems; it is not a universal test standard for every kind of film.
The ISO record lists ISO 6:1993 as published in February 1993 and says it was reviewed and confirmed in 2023. The public abstract describes the standard’s purpose, but not the full test procedure or its detailed measurement conditions.
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What does a higher or lower number mean?
A higher speed rating means the film responds faster to light. That can help when shooting in lower light or photographing a moving subject, because you can use less light for a given exposure. Slower-rated film can be suitable in brighter conditions or when photographing subjects that move slowly.
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The rating does not determine exposure by itself. Exposure also depends on how much light reaches the film and for how long, as controlled by the scene and camera settings.
How far apart are ISO 100 and ISO 200?
ISO 100 and ISO 200 are one stop apart: ISO 200 is twice the speed rating of ISO 100. A stop represents a doubling or halving of exposure. Kodak uses these two ratings as an example of a one-stop difference.
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What does “ISO 100–400” mean?
It usually indicates a range of film-speed ratings from ISO 100 to ISO 400, rather than one fixed rating. ISO 400 is two stops faster than ISO 100: the rating doubles from 100 to 200, then doubles again from 200 to 400. The range alone does not identify the film type or establish its grain, sharpness, or overall image quality.
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No. ISO 6 covers black-and-white pictorial still-camera negative film and process systems. ISO identifies separate standards for color negative still film (ISO 5800) and color reversal camera film (ISO 2240). When comparing films, consider both the speed rating and the film type, since the applicable standard depends on the material and use.
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How is ISO speed different from exposure index (EI)?
ISO speed is a standardized rating. For Kodak motion-picture film, the manufacturer’s stated exposure index (EI) is a recommended exposure value derived from practical picture tests. Kodak says its EI recommendations account for normal variation in film and equipment, and are not absolute sensitivity measurements or values that can never change.
For that reason, a motion-picture stock’s stated EI should be described as the manufacturer’s recommended exposure index, not automatically as an ISO speed. The two labels are not interchangeable in every context.
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Does a higher ISO rating mean more grain or worse image quality?
Speed alone is not a quality score and does not determine grain, sharpness, or image quality. Grain can matter when choosing film, but the available official guidance does not establish a universal numerical relationship between ISO speed and grain. Compare a specific film’s characteristics and intended use rather than treating its ISO number as a complete quality rating.
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