reciprocity for bergger bpf 200 shot @100

Discussion in 'Exposure Discussion' started by toddstew, Nov 15, 2005.

1. toddstewMember

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Does anybody have any experience with reciprocity failure with bergger bpf 200 shot at 100asa? If so, do you happen to know the correction times? Or can someone direct me to the times?
Thanks,
Todd

2. jandcMember

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To 1/2 second - no adjustment

1 second - 1.5 stops

10 seconds - 3 stops

100 seconds - 4 stops

3. toddstewMember

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thanks jandc. Pardon my dumb question, but how would that transfer to time? 1.5 stops equals how much time increase if I want to keep the same f-stop? Would 10 seconds become 30 seconds?

4. TheFlyingCameraMembership CouncilCouncil

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each additional stop is a doubling of the time - so yes, 10 becomes 30 seconds.

5. toddstewMember

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Thank you for the illumination(yuk,yuk)
Todd

6. Jim NoelMember

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Oops ther is a math error here.
If the exposure is 10 seconds a simple calculation would be 40 seconds (10x2=20;20x2=40)
However, I believe you will find that amount of time insufficient because the reciprocity for 40 seconds now comes into play. 3 stops is 2 raised to the third power, or 8. At the very least then, the time requirted would be 80 seconds.
To find out for sure, make 3 exposure, one at 40 seconds, one at 80 and one at 160 seconds. These are one stop apart and yuo will learn a lot about the reciprocity of the film. In fact, if you have a densitometer you will have enough information to make a graph of the reciprocity curve from which you can calculate the reciprocity for any given amount of time.

7. toddstewMember

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thanks jim, I'll try it.

8. chrisofwlpMember

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Opps one more math error. A ten second second exposre increased by three stops results in an 80 second exposure. Metered timeX2^number of stops=correct exposure. 10x2^3=80. This formula almost never fails.

Christopher Breitenstein

Last edited by a moderator: Mar 1, 2007
9. bruce terryMember

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Am I crazy? Look at ilford's hp4 reciprocity specs here>http://www.ilfordphoto.com/Webfiles/2006216115141521.pdf. A metered 10 secs is recommended to be adjusted to about 30 seconds. According to their chart this mean "times 3" ... not a quantum ride of additional 3 "stops", or 10>20>40>80 sec.

Haven't done dark light above 8 seconds metered, but about x3, or 24 sec of exposure works fine at that level of the curve. Sure would like to know how far out in left field and/or lucky I am!! ;]

Bruce