BetterPhoto Q&A
Category: New Answers

correctly exposing, using a long exposure time

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March 12, 2007

The published film speed or ISO is generally valid for shutter speeds between 1/10 thru 1/1000 second. Should the photographer choose to expose using an exceedingly fast or enormously slow shutter, film speed plunges.

Changes in film speed (response / sensitivity) due to changes in shutter speed is know as the Schwarzschild effect or more commonly, Reciprocity Departure.

The law of reciprocity tells us that the amount of “blacking” (density produced) is unchanged with exposure time so long as the total exposure remains unchanging per the equation E=IT (exposure = intensity x time. The law rarely holds.

Trial correction factors:
Astronomers often choose long exposure times. It is wise to test the material to determine what the revised ISO will be at extended times however; the following correction values will be helpful.

1 minute = 1.5 -- 2 minutes = 2 – 4 minutes = 3 – 8 minutes = 4.5 – 10 minutes = 5 – 15 minutes = 6.33 – 20 minutes = 6 – 30 minutes = 7 – 40 minutes = 7 – 60 minutes = 8

Assume your film is 800 ISO and you choose to exposure for 10 minutes. The revised ISO will be 800 ÷ 5 = 160.

To correct for long exposure:
Determine exposure time exclusive of any correction. Say you conclude it will be 4 minutes. Now multiply this time by the correction factor. The correction factor for 4 minutes is 3. Thus 4 minutes x 3 = 12.Twelve minutes is the trial exposure time for this circumstance.

Alan Marcus
Ammarcus@earthlink.net

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March 13, 2007

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March 14, 2007