500 Rule Calculator
Max exposure to limit star trails.
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The 500 rule
A traditional rule of thumb for untracked night sky photos: maximum shutter speed in seconds ≈ 500 ÷ effective focal length (focal length × crop factor). Longer exposures show star trails.
Formula
t_max ≈ 500 / (f × crop)
FAQ
Why do my stars still trail?
High-resolution sensors reveal motion earlier. Try the stricter NPF rule calculator, or shorten exposure and raise ISO / use a tracker.
Night-sky practical tips
Pointing near the celestial equator increases trailing versus near the pole. Use the 500 rule as a first guess, then inspect at 100% on a short test exposure. Trackers allow much longer times; turn tracking on and re-think exposure around skyfog and star color rather than trailing alone.