Astrophotography Exposure Calculator
The Earth rotates — expose too long and stars become streaks. Find your maximum shutter speed with the classic 500 rule, the stricter 300 rule, and the modern pixel-level NPF rule.
The three rules
500 rule: shutter = 500 ÷ (crop factor × focal length). Quick, but derived for film and prints — stars often trail slightly on modern high-resolution sensors viewed at 100%.
300 rule: the same formula with 300, a conservative variant for sharper results and larger prints.
NPF rule: shutter = (35 × aperture + 30 × pixel pitch) ÷ focal length, where pixel pitch is in microns. It accounts for your actual sensor's pixel size, so it's the most accurate for pixel-level sharpness. Pixel pitch is derived here from your sensor width and megapixels (3:2 frame assumed).
Frequently asked questions
The NPF time is very short — what can I do?
Raise ISO (3200–6400 is normal for astro), open the aperture, use a wider lens, stack multiple shorter exposures, or use a star tracker to eliminate the limit entirely.
Does the direction I point matter?
Yes. Stars near the celestial equator move fastest; near the celestial pole (Polaris) they barely move. These rules assume the worst case, so you gain margin pointing toward the pole.
What settings work for the Milky Way?
A common starting point on full-frame: 14–20mm, f/1.8–f/2.8, ISO 3200–6400, and the NPF time from this calculator. Focus manually on a bright star using live view at maximum magnification.
Last updated: 2026-07-12