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07-17-2009, 01:16 PM   #1
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Exposure Timing

I just read an article about night photos. The author said to take a 30 second test shot at 3200 ISO. If it comes out right then use an 8 minute exposure at ISO 200. Now I'm not a math wiz by any means but what is the formula? or is this trial and error sort of thing?

07-17-2009, 02:12 PM   #2
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By calculation, this is correct (ISO 3200 is 4 stops of light more sensitive than ISO 200, and 30 seconds is 4 stops of light less exposure time than 8 minutes), but I'm not sure the 'test' shot settings are necessary - just depends what you want to capture.

If you're just after getting the stars to come out, I believe it's more important to get the aperture and sensitivity settings right, and the shutter speed can be left to however creative you want to be with the star trails. A 30 sec exposure at the right Av/ISO setting will still get the stars to come out, and a 30 minute exposure at the same Av/ISO setting will then get the stars trailing in the sky. Note that this only applies if there is no other ambient light falling on anything in the frame to interfere with exposure. So again I'll repeat that it depends what you want to capture in the night shot.

Hope this helps.
07-17-2009, 03:06 PM   #3
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This is how to extrapolate 8mins at ISO from 30 secs at ISO 3200:

30 seconds at ISO 3200
60 seconds at ISO 1600
120 seconds at ISO 800
240 seconds at ISO 400
480 seconds at ISO 200
960 seconds at ISO 100

These figures will all give you the same exposure. This is because each stop of decrease in ISO halves the sensitivity and you have to double the exposure time to compensate to keep the exposure the same. (Can also be done by opening up the aperture by one stop).
07-20-2009, 05:05 AM   #4
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Exposure Settings

Thanks for the information. Makes sense to me now.

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