Find the maximum shutter speed for sharp stars — with the rule of 500, the stricter rule of 300 and the NPF formula.
The rule of 500 is a rule of thumb for astrophotography: divide 500 by the equivalent focal length (focal length × crop factor) to get the maximum shutter speed in seconds before stars start to trail. At 14 mm on a full-frame sensor, that gives about 35 seconds. At 200 mm, about 2.5 seconds.
The rule of 300 is a stricter version that produces sharper stars on high-resolution sensors. And the NPF formula, developed by the Société Astronomique de France, takes pixel density into account: NPF = (35 × crop + 30 × √megapixels) / focal length. It gives shorter times than either rule but produces genuinely point-like stars on modern sensors.
All three are approximations. The real answer depends on the declination of the stars (stars near Polaris move less), the final print size and your tolerance for trailing. Use the NPF value as a starting point and adjust.
Rule of 500: t = 500 / (focal length × crop factor). Rule of 300: t = 300 / (focal length × crop factor). NPF: t = (35 × crop + 30 × √MP) / focal length.
The NPF formula is the most accurate on modern high-resolution sensors. The rule of 500 is fine for social media, where pixel-peeping is not a concern.
A star tracker compensates for Earth's rotation, so these rules do not apply — you can expose for minutes instead of seconds.
Yes — a crop sensor at 14 mm is equivalent to about 21-22 mm on full frame, and the stars trail faster because the field of view is narrower.
Vai.la turns any URL into a short link with click statistics, QR Code and your own biolink.
Vai.la is not responsible for how the tools are used or for decisions made based on their results.