how many stops different are a 100 ISO film stock and an EI320 film stock , and which is faster?


If you are wondering what the difference is between ISO and EI, these are used in different contexts.

ISO gives the speed of the film.

EI stands for exposure index, and it's the speed that you have used to expose a given film. Most of the time you will use an exposure index that matches the ISO rating of the film. If you want to "push" a film for example, you might "rate" ISO 400 film at 800, i.e. use an exposure index of 800, or expose ISO 400 film at EI 800. This then is taken into consideration when processing the film.

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The difference between ISO 100 and ISO 320 is 1⅔ stops. If you shoot ISO 100 film at EI320, you have underexposed it by 1⅔ stops and it will need to be "pushed" by 1⅔ stops when developed.

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EI320 is sort of misnomer when attributed to film stock. For a film that is not yet exposed ("film stock") one can say "it is REI320"; meaning the recommended exposure index (REI) is as if the film stock speed is ISO 320. EI reefers to exposed film (and exposed film is not exactly "stock").

In processing practice the difference between ISO 100 and EI 320 is not as simple as log2(320/100). It depends on what the ISO speed was the film stock, and what is the intended purpose of EI. If the purpose is having a denser negative, and the original ISO is 400, no adjustment in development is made, the film is processed as ISO 400. Usually for an ISO 400 film EI may be as low as 200 without any adjustment of film development. If the purpose is to have a lighter negative, and we start with ISO 100 film, exposing it as ISO 320 (EI 320), we may push 1 stop in processing, and the difference between ISO 100 and EI 320 becomes 2/3 of a stop instead of 1 2/3.

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