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What is happening with 35mm film after expiration date? Will the ISO drop, so for example 200 ISO film one year after expiration date will be at an equivalent ~100 ISO?

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  • \$\begingroup\$ What film? How was it stored? \$\endgroup\$
    – user82065
    Jul 22, 2019 at 7:52
  • \$\begingroup\$ The biggest issue with color print film is normally color shift. If stored at room temperature, one year after expiration, you may hardly notice the difference. After 5 years, it may be noticeably degraded, but you should be able to fix it digitally. After 10 years, you may not be able to fix them completely, but if they are important images, try anyway. These are my experiences, your results may vary. Storage temperature is critical. \$\endgroup\$
    – Mattman944
    Jul 22, 2019 at 13:17
  • \$\begingroup\$ @tfb I bought it from eBay c200 expired for 1.6 year. \$\endgroup\$
    – Andy Andy
    Jul 22, 2019 at 21:15
  • \$\begingroup\$ @mattman944 thank you. I am looking in color shift and I'm thinking to bake it as well just to get an odd/old color palete. \$\endgroup\$
    – Andy Andy
    Jul 22, 2019 at 21:18

2 Answers 2

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Each light sensitive silver salt crystal that comprises film emulsion has an exposure threshold. We are talking about the amount of light energy required to render any given crystal developable. The ISO for any given material is an average of the communal sensitivities. The threshold value is variable. It is alterable due to the activity of the developer and a dozen other factors.

As time goes by, outside forces such as heat, pressure, ionizing radiation and chemical instability shove the crystal towards its threshold. Another way to say this, films and photo papers are constantly being fogged. In time, the material becomes useless as it will develop up just as if it had been exposed to light.

Now many fast films are hyposensitized at the factory. Suppose a recipe produces 400 ISO film. The maker might hyposensitize some and make a batch that is1600 ISO. This is done by chemical treatment or a controlled fogging to light or other radiation. Astronomers routinely treated film by baking in a container filled with a mixture of different gasses.

What I am trying to say is; aging film will boost its ISO. The problem is, aged film is unpredictable. It may be fogged beyond hyposensitized thus to overcome the fog, you must give the material more exposure. Also, be aware, color films contain multiple emulsion layers; these are independent as to the way they age. Thus color film undergoes a color balance shift as it ages.

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    \$\begingroup\$ It's not the case that film gets faster as it gets old: it gets less fast. There are various rules of thumb but I have always used a stop per decade for well-stored film (so 10-year-old film with a box speed of 400 I might expose as if it was 200). \$\endgroup\$
    – user82065
    Jul 22, 2019 at 16:53
  • \$\begingroup\$ I stand by what I said above -- shelf time hyposensitizes. A. Marcus \$\endgroup\$
    – Alan Marcus
    Jul 22, 2019 at 20:54
  • \$\begingroup\$ @AlanMarcus thank you. So if I heat a roll in the oven, will I increase the ISO? \$\endgroup\$
    – Andy Andy
    Jul 22, 2019 at 21:23
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    \$\begingroup\$ I guess I'm just imagining all the many rolls of old film I've exposed at a couple of stops under its rated speed then. Oh well. Also, your answer is just internally incoherent: you talk about 'boosting ISO' but 'hyposensitizing': hyposensitize means to make less sensitive: to reduce the ISO! \$\endgroup\$
    – user82065
    Jul 22, 2019 at 21:49
  • \$\begingroup\$ Hypersensitized is the word I should have used - to elevate the speed of a film. Done by chemical action - pre-exposure to light or other radiation. Astronomers use a mix of gases and bake. Also, freeing film (cold pack) limits reciprocity failure. Alan Marcus \$\endgroup\$
    – Alan Marcus
    Jul 22, 2019 at 23:05
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At the very basics, that depends on how it was stored. let's say it was stored dry and frozen (best) conditions: It will take good quality pictures for many years, or even decades, past the expiration date. But if it was warm and humid, then it would take washed-out and grainy pictures.

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