Astrophotography Exposure (500 & NPF Rules)
Longest untracked star exposure before trails show, by the 500 and NPF rules, from lens, aperture, sensor and pixel count, with declination and trail rotation.
The 500-rule (or your constant) and NPF exposure limits, the declination correction for where you aim, the image scale in arcseconds per pixel, and how far the sky turns during a star-trail session.
Example: 24 mm at f/2.8 on a 24 MP full-frame camera: the 500 rule allows 20.8 s, the NPF rule 11.6 s; three hours of star trails turn the sky 45°.
Before the stars
start to move.
What the two rules estimate and why they differ, what declination changes, and how far the sky turns for star trails.
The 500 rule
The oldest rule divides a constant by the focal length times the crop factor: 500 ÷ (24 × 1) ≈ 21 s. It was made for film and small prints; on today’s dense sensors stars trail visibly at 100 % view well before that, which is why many use 300 or 400 — the constant is an input.
The NPF rule
Frédéric Michaud’s NPF rule (Société Astronomique de France) brings in the aperture and the pixel pitch: in its simple form t = (35·N + 30·p) ÷ f seconds, with p in microns. Smaller pixels and longer lenses shorten the limit; it is usually the one to trust on modern cameras.
Declination and trails
Stars near the celestial pole move less than those on the celestial equator, so both limits may be divided by cos δ for the declination you aim at; 0° is the strict case. For star trails the sky turns 15.04° an hour, so a three-hour session sweeps 45° of arc around the pole.
What the number does not settle
Both are empirical rules for points that look like points at normal viewing; a tracking mount removes the limit, and sky brightness and noise decide the rest. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.
SOURCES
- NPF rule — Frédéric Michaud, Société Astronomique de France (simplified form)
Last reviewed 24 September 2026. How results are checked: How we verify.