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I just realized that I shot a roll of Portra 160, while my camera was set on iso 400.

Is there something I should ask for at the developing lab to make up for it?

I hope that some of you might be able to help me out - thanks!

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Your camera "thought" it had film with an ISO of 400, while in reality it was only 160. So it adjusted it's exposure meter for ISO 400 film, underexposing your film by a bit more than 1 stop.

To compensate for this underexposure you need to ask the lab to "overdevelop" your film by 1 1/3 to 1 1/2 stops. This is called push processing.

I have no experience with this process, I just gathered the information from existing answers.

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    To be precise, ISO 400 is 1.3 stops faster than ISO 160, so OP should request to have the development pushed by about 1 1/3 stops. – scottbb Apr 26 at 22:29
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    And OP - do be precise. Color Negative film (like Portra) does NOT look good underexposed. It was designed with overexposure in mind. Push processing will get you usable negatives (and probably good ones to boot!) - regular processing of underexposed film will give you grey, dull, junk negatives to work with. – OnBreak. Apr 27 at 7:09

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