Work out star trail exposure instantly with clear inputs, formula shown and shareable results.
The 500 rule sets the maximum exposure at 500 divided by the equivalent focal length, and dividing further by the cosine of declination allows for slower apparent motion near the poles. Stars move 15 degrees an hour at the celestial equator but almost not at all near Polaris, which is why northern shots tolerate much longer exposures.
500 rule
seconds = 500 / (focal length x crop factor)
Declination allowance
seconds = 500 rule / cos(declination)
seconds = 500 / (focal length x crop factor). The 500 rule sets the maximum exposure at 500 divided by the equivalent focal length, and dividing further by the cosine of declination allows for slower apparent motion near the poles.
Stars move 15 degrees an hour at the celestial equator but almost not at all near Polaris, which is why northern shots tolerate much longer exposures.
Enter focal length, crop factor, target declination. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.