I shoot 35mm film for fun.
I have received a pack of Kodak ColorPlus 100 iso, expiration 2001. How many f stops should I go?
The film was kept in good conditions (shaded and cooled).
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You're in luck. About two decades past expiration isn't terribly long, especially if stored well. Fog is worse on faster film than slower, so you're in luck here as well.
And finally, it's color negative film - which is designed to tolerate overexposure of many (up to 6 in some films!) stops.
I's personally rate it at 1 stop down (ISO 50) and go from there. At worst, it gets you closer to the film's aged and slower speed. At best, the film hasn't suffered much and this offers a slight over-exposure (which is not a bad thing at all with color negs).
If you have multiple rolls, consider shooting some tests.
ColorPlus 100 is a negative color film. Negative films are a means to an end. In other words, negative film is designed to be transformed to a positive image for viewing. The negative / positive process of photo imaging is very forgiving as to the necessity to get the exposure “right”. Originally we re-exposed the developed negative film by projecting its image onto light sensitive photo paper. This exposed paper was then developed and a positive print resulted.
You might want to ask yourself, why was the negative / positive process so popular when other film types yielded positive (slides) images from the get-go? The answer is, the printing process was virtually a re-taking of the original picture. We exposed the picture again onto photo paper. This second re-taking afforded substantial opportunity to correct any errors such as over / under exposure and or errors due to improper developing. We are talking “latitude”. Slide film has narrow latitude, under an f-stop. Whereas, negative film are able to tolerates several f-stops of inaccuracy.
In modern times, we likely skip the making of chemical based prints by utilizing computer based hardware and software make viewable positive images on screen or prints on paper. These modern methods expand film “latitude” in ways never dreamed of by its chemical based designers.
In other words, don’t worry about ageing film changing ISO. Just expose using settings based on box speed.
That being said, most will advise you to use a lower ISO setting under the theory that old film suffers a loss of speed. Contrary to this accepted view, my experience is, older films gain speed so I do the opposite, elevating the ISO by about ½ f-stop. My theory, aged film has an increased fog level. It’s light sensitive goodies have moved closer to the exposure threshold due to exposure to ionizing radiation (always present) and chemical instability.
Anyway, it likely makes no difference if this aged film is faster or slower, the negative / positive scheme makes this moot because this film has wide “latitude”.
What is not mentioned in any of the other answers: Don't expect great results, no matter how you adapt the exposure.
Black and white films can ofter be used decades past expiration if you adapt exposure and development, but colour films, both negative and slide, are much more prone to degradation. The chemicals embedded in colour films, which form the basis to build dyes during the development process, are much less stable than the silver halides used in black and white film to form the image. Colour couplers for different base colours also degrade at a different rate, so you are very likely to experience colour shifts, which very unlikely can be fixed when traditionally printing the negatives and also rather unlikely can be fixed in digital post-processing if you scan the negatives. I don't know exactly why, but my experience is also, that expired colour films soon tend to get a much more prominent grain structure.
Both the shifted colours and the increased grain can of course be used on purpose to get a specific 'look', but you honestly can't expect the film to give similar results now compared to 20 years ago.