What's the difference between the 500 rule and the NPF rule for astrophotography?

The 500 rule is one division: 500 over your full-frame-equivalent focal length, nothing else — a quick, camera-agnostic estimate inherited from the film era. The NPF rule takes your actual camera's pixel pitch into account, and in its complete form the declination of whatever you're photographing too, which is why it lands on a genuinely more accurate shutter-speed ceiling before trailing shows up at full resolution.

The Gap Grows With Resolution

On a 24-megapixel full-frame body at 24mm f/2.8, the two rules land at roughly 21 seconds (500 rule) against roughly 12 seconds (NPF) — already a meaningful gap. On a 61-megapixel body at the same focal length and aperture, the NPF figure shortens further, to roughly 9 seconds, while the 500 rule's answer doesn't change at all, since it has no resolution term to respond to. The higher your camera's resolution, the less the 500 rule's estimate can be trusted at face value.

Neither Number Is Simply Wrong

Think of them as answering slightly different questions rather than one being simply outdated. The 500 rule is calibrated for a final image judged at normal viewing size or a moderate print; the NPF rule is calibrated to survive someone zooming into the actual pixels — a bigger deal the more your workflow involves heavy cropping or large-format printing. Which one to trust comes down to what you're actually going to do with the file, not which formula wins in the abstract.

See both numbers side by side for your own camera and lens in the Astro 500/NPF Rule Calculator.