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I've accidentally exposed two 400 ASA B&W films at 200 ASA. One I developed like a 400 and the negative has come out very under exposed. Any advice on how I can salvage the other one?

Thanks

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    Are you sure about this? Exposing for sensitivity one stop lower should result in overexposure. – mattdm Apr 19 '16 at 11:57
  • @StephanieWestonSmith What film is it? Underexposing is not uncommon even for 400ISO B&W films... Depending on which film it is and what developer you are using, you can still get some very decent and usable images out of it. You might want to check www.filmdev.org for a specific recipe to develop your film. – jrojasqu Apr 19 '16 at 14:25
  • If the negative is too dark that means it is overexposed. It's a negative, after all, not a positive. An underexposed negative will be too light. – Michael C Apr 19 '16 at 20:30
  • Ah, yes, maybe what Michael says. Stephanie, can you clarify? – mattdm Apr 21 '16 at 1:50
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    @mattdm The negative came out too light – Stephanie Weston Smith Apr 21 '16 at 10:59
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I've accidentally exposed two 400 ASA B&W films at 200 ASA

In other words, you've overexposed it by a stop. (See What is the relationship between ISO, aperture, and shutter speed? if you need a refresher on that.) The fix for this is called "pull processing" (the opposite of "push processing", which is used when you underexpose, treating the fill as one rated for a higher sensitivity.

In general, this isn't recommended, as it often reduces contrast and gives a "murky" overall look, but having done it, it's probably your best option. The information for your chemicals should give you recommended pull processing times.

The puzzlement, though, is that you say you developed the film as normal and it came out under exposed. This is surprising, because that's the reverse of what would be expected in this situation.

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