500 Rule
The 500 rule divides 500 by your full-frame-equivalent focal length to estimate the maximum shutter speed before star trailing becomes visible — a fast, camera-agnostic mental-math shortcut from the film era, still useful as a rough field estimate today. At 24mm on full frame, it suggests roughly 21 seconds; at 14mm, roughly 36 seconds.
The rule has no resolution term, so it gives the identical answer for a 12-megapixel and a 100-megapixel camera at the same focal length, which is exactly why it increasingly understates real trailing risk as sensor resolution climbs. The more accurate NPF rule, which factors in pixel pitch and declination, closes that gap for any specific camera.
Frequently Asked Questions
Is the 500 rule still useful given more accurate alternatives exist?
Yes, as a fast estimate when you don't have your camera's pixel pitch handy, or when the final output is small enough (web, social media) that pixel-level trailing accuracy doesn't matter as much.
What are the 300 rule and 600 rule?
Stricter and looser variants of the same fixed-divisor approach — 300 builds in extra safety margin for closer scrutiny, 600 is more generous for small final output — both share the identical resolution-blind limitation as the 500 rule.