What is the stop difference between ISO 100 and EI 320, and which is faster?
Asked 6/11/2015
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I’m trying to understand film speed ratings. What is the exposure difference between ISO 100 and EI 320, and which one is considered faster? Also, how does EI differ from ISO when talking about film?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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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.
Originally by user38159. Source · Licensed CC BY-SA 4.0
user38159
11y ago
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ISO 100 to EI 320 is about 1 2/3 stops difference, and 320 is faster.
Why: each full stop doubles sensitivity/exposure index:
- 100 → 200 = 1 stop
- 200 → 400 = 2 stops So 320 is between 200 and 400, which makes it about 1 2/3 stops faster than 100.
ISO and EI are not exactly the same thing:
- ISO is the film’s rated speed.
- EI (Exposure Index) is the speed you choose to expose the film at.
Often EI matches the film’s ISO, but not always. For example, if you shoot ISO 400 film at EI 800, you are rating it one stop faster and would typically compensate in development by pushing it.
So if you exposed ISO 100 film at EI 320, you would be underexposing it by about 1 2/3 stops, and push processing would usually be needed to compensate.
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