Free tool
NPF Rule and 500 Rule Calculator
How long can you expose before stars start to trail? Enter your sensor, megapixels, lens and aperture. The calculator gives the NPF rule time for sharp stars and the 500 rule time for comparison.
The NPF rule
The NPF rule was published by Frédéric Michaud of the Société Astronomique du Havre. The simplified version is: shutter time in seconds = (35 × f-number + 30 × pixel pitch in µm) ÷ focal length in mm. Pixel pitch is the size of one pixel on the sensor. The calculator works it out from the sensor size and megapixels. A high-resolution sensor has smaller pixels, so trailing shows sooner and the time is shorter.
The 500 rule
The 500 rule divides 500 by the full-frame equivalent focal length. It comes from the film era and gives times that are too long for modern sensors: stars look sharp on a phone screen but show short trails when you zoom in or print. Use it for quick shots for small screens, and the NPF rule when you want pinpoint stars.
Why the direction matters
Stars near the celestial equator move fastest across the frame. Stars near the pole move much more slowly, so you can expose longer when you aim north (or south in the southern hemisphere). The calculator corrects both rules for where you aim, using the same formula as the Still Dark app (time ÷ cos of the declination, capped at 60×).
Treat the result as a starting point. Take a test frame, zoom in on the corners, and shorten the time if the stars are oval.
Still Dark Camera Tools works this out for the camera and lens you've saved, along with depth of field, hyperfocal distance, star trails, time-lapse and settings for the Milky Way, Moon, meteors and eclipses (Pro).
