# RECIPROCITY ADJUSTMENT

Discussion in 'Exposure Discussion' started by Bruce Osgood, Apr 22, 2006.

1. ### Bruce OsgoodMembership CouncilCouncil

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Occasionally I find my brain frozen in the dumb zone.

If you have an indicated exposure of 45 seconds and want to account for reciprocity that is stated as:

1 sec: expose 2 secs, (-develop by 10%)
10 secs: expose 1 min (-develop by 20%)
100 secs: expose 20 mins (-develop by 30%)

...would you use the 10 second exposure four and one half times (4-1/2 minutes)?

TIA,

2. ### BradSMember

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I'd think of it like this:

at 1 second, they're asking you to apply a factor of 2x. At 10 seconds a factor of 6x and at 100 seconds the factor grows to 12x.

45 seconds is kinda in the middle of 10 and 100 so, I might try a factor of 8x or 10x

that would mean, at 45seconds, try 6 - 7.5 minutes..

3. ### Bruce OsgoodMembership CouncilCouncil

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HMMM, I never thought of it quite like that. Interesting. This could explain a lot of things to me.

Thanks Brad.

4. ### BradSMember

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Hey Bruce,

It's no trouble at all. I was just thinking about this a little more and, if we take just the last two points, that is at 10 seconds and at 100 seconds, we see that the corrected exposure time can be expressed as follows:

y = (6* x * log10(x)) / 60

where:
y is the exposure time corrected for reciprocity failure in minutes and,
x is the base exposure time in seconds

if you apply this to your original question, the exposure time, adjusted for reciprocity is about 7.5 minutes...

try it:

y = (6.0 * 45 * log10(45)) / 60.0 == 7.439

Brad.

5. ### krisbfunkMember

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e100G transparency - reciprocity calculation?

Hey.. since you're on the topic . . .

I'm shooting a matchbox pinhole with Ektachrome e100G transparency film. I'm just wondering how to calculate the exposure times for reciprocity.

Kodak's website states for e100G:
"Outstanding reciprocity - No speed or color compensation required for exposures from 1/10,000 to 10 seconds"

ok, but what's the calculation after 10 seconds.. is there a universal calculation? or is it different for each type of film.

sorry for the newb question.

6. ### cdholdenMember

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As per Kodak,
http://www.kodak.com/global/en/professional/support/techPubs/e4024/e4024.jhtml#adjustmentsforlongandshortexposures

"No filter correction or exposure compensation is required for exposure times from 1/10,000 to 10 seconds. At exposure times of 120 seconds, add CC10R filtration."

Each film reacts differently. Many need longer exposure. This one appears to only need color correction. Some need both.
HTH
Chris

7. ### krisbfunkMember

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ok, that registers. thanks for the info.