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I know to create a star trail image it is possible to stack many images taken over a period of time. However I recently saw an image which had been done as a single exposure of 1 hour.

How do you calculate such long exposures?

Typically for astrophotography you use the widest aperture and high ISO to reduce shutter time. My typical settings would be:

f3.5, iso6400, 13s exposure.

If I change to f28 and iso100 how do I determine the correct exposure time?

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f/28 and ISO 100 is NOT what you want to do for star trails. For one thing, Depth of Field is unimportant when focused on stars. For another, the night darkness may not be that long. :) ISO and aperture exposes the star trail brightness, and time then determines the length of the rotation arc (360 degrees in 24 hours).

Stacking minimizes the ISO noise that accumulates in one long exposure.

But a general answer to your specific question is that there is a calculator that might be helpful to compute any such exposure questions, at http://www.scantips.com/lights/exposurecalc.html

Perhaps you meant f/2.8 ?

One choice for your f/3.5, ISO 6400 and 13 seconds says f/2.8 and ISO 100 is 512 seconds, just barely in range of the calculator.

Equivalent is not what this question wants though. Again, ISO and aperture exposes the star trail brightness, and time then determines the length of the rotation arc (360 degrees in 24 hours).

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