The 500 Rule for Astrophotography
How to keep stars sharp in a nightscape, and when to switch to the stricter 300 or NPF rules.
Open the calculator: 500 Rule Calculator →
Point a camera at the night sky with a long exposure and the stars record as short streaks, because the Earth is turning. The 500 rule is a fast way to find the longest shutter speed that still renders stars as points: divide 500 by your full-frame-equivalent focal length, and the answer is the maximum time in seconds. A 20 mm lens on full frame gives about 25 seconds.
500, 300 or NPF?
The 500 rule is a friendly starting point, but modern high-resolution sensors reveal trailing sooner, so many shooters use the stricter 300 rule. The NPF rule is more precise again, taking aperture and pixel pitch into account, and typically recommends shorter times for genuinely pinpoint stars. Use 500 for a quick estimate and 300 or NPF when you pixel-peep.
Why focal length and sensor matter
Longer lenses magnify star movement, so they demand shorter exposures. Crop sensors do the same through their equivalent focal length, which is why the rule uses the full-frame equivalent. To keep exposures short and stars sharp, pair a fast wide-angle lens with a high ISO, and consider a star tracker for longer, cleaner shots.
Getting a clean shot
Shoot in raw, focus manually on a bright star using magnified live view, and take several frames to stack later for lower noise.
Open the calculator: 500 Rule Calculator →