How do you calculate equivalent very long exposures for star trail photography?
Asked 2/5/2017
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I know star trails are often made by stacking many shorter exposures, but I recently saw one done as a single 1-hour exposure.
For example, if my normal night-sky exposure is around f/3.5, ISO 6400, 13 seconds, how would I calculate an equivalent longer exposure after changing aperture and ISO? For instance, what would happen if I changed to ISO 100 and a much smaller aperture?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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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).
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
9y ago
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Generated from our catalog & community — verify before relying on it.
Use exposure equivalence: each full stop less light from aperture or ISO requires doubling the shutter time.
Starting from f/3.5, ISO 6400, 13s:
- ISO 6400 to ISO 100 is 6 stops less sensitivity, so shutter time must be 64× longer.
- f/3.5 to f/2.8 is about 2/3 stop more light, so compared with f/3.5, f/2.8 would need less time; one example given was about 512s at f/2.8, ISO 100.
For star trails specifically, though, f/28 and ISO 100 is generally not desirable. Aperture and ISO mainly control trail brightness, while exposure time controls how long the trails become. Since stars are effectively at infinity, depth of field is not the reason to stop down heavily. Very long single exposures also build up more sensor noise, which is why stacking many shorter exposures is commonly preferred.
A practical way to calculate equivalents is to use an exposure calculator and count stops between your settings.
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