Astro 500/NPF Rule Calculator
How to Read This Result
Both numbers answer the same question — how long can you leave the shutter open before stars stop looking like points and start looking like short streaks — but they get there very differently. The 500 Rule number is a rough, camera-agnostic estimate: divide 500 by your full-frame-equivalent focal length. It was a reasonable mental-math shortcut in the film era and still gives a usable ballpark today, but it doesn't know anything about your specific camera's resolution.
The NPF Rule number accounts for your camera's actual pixel pitch (smaller, denser pixels reveal trailing sooner, so a high-resolution camera needs a shorter exposure than the 500 rule would suggest) and — when you adjust the declination slider — the fact that stars near the celestial equator appear to move faster across the sky than stars near the celestial pole, so the same shutter speed that keeps a polar-region star sharp may show visible trailing on a star near the equator.
Why the Two Numbers Disagree
On a modern high-resolution camera, the NPF number is usually noticeably shorter than the 500 Rule number — the higher pixel density means trailing becomes visible at a shorter exposure than the older rule accounts for. On an older or lower-resolution camera, the gap narrows. Neither number is "wrong" so much as answering a slightly different question: the 500 Rule targets "looks fine at normal viewing size," while the NPF Rule targets "looks like a clean point at typical pixel-level scrutiny."
The NPF Formula
This calculator uses the commonly published simplified NPF formula: t = (35 × N + 30 × p) / f, where N is your f-number, p is your pixel pitch in microns, and f is your actual (not full-frame-equivalent) focal length in millimeters. This is a widely used approximation, not a guaranteed-precise physical constant — different sources publish slightly different coefficients, and this is stated here rather than presented as more exact than it is.
Pixel pitch itself is computed live from the megapixel count and sensor format you enter — the calculator derives it from your sensor's real physical width and your camera's real resolution rather than asking you to look up a spec sheet value yourself. Look up your specific camera on the camera reference to get its exact megapixel figure if you're unsure.
Want the full explanation behind this calculator? Read the guide.
Frequently Asked Questions
Should I trust the 500 rule or the NPF rule?
The NPF rule is more accurate for modern high-resolution cameras and is the better starting point if you plan to view your images at full resolution or make large prints; the 500 rule remains a fine quick mental estimate in the field if you don't have your camera's pixel pitch handy.
Do I still need to worry about star trailing if I'm using a star tracker?
No — a star tracker rotates the camera to follow the sky's apparent motion, which is specifically what defeats the trailing this calculator predicts. Both the 500 rule and the NPF rule assume a stationary tripod.
Why does declination change the result so much near 90 degrees?
Stars very close to the celestial pole barely appear to move at all over a normal exposure, so the safe exposure time genuinely does increase dramatically as declination approaches 90 degrees — this isn't a calculator quirk, it reflects the real geometry of how the sky appears to rotate.