Star Photography Calculator
NPF-rule sharp-star exposures, and the drift math for deliberate star trails.
How it works
The sky moves 15 arcseconds every second. How long you can expose before a star smears into a dash depends on focal length, aperture, pixel pitch and where in the sky you point — the NPF rule accounts for all four, where the old 500 rule only knew focal length. Pick your camera and lens for the longest sharp-star exposure, or flip it around: choose a trail length and see how many frames to stack.
FAQ
- Why is NPF shorter than the 500 rule?
- The 500 rule dates from film and low-resolution sensors. Modern 24–60 MP sensors resolve trailing the 500 rule never saw; NPF (Michaud, Société Astronomique du Havre) uses aperture and pixel pitch to predict where the smear actually becomes visible.
- What declination should I enter?
- 0° for the celestial equator (Orion's belt, most of the Milky Way core region at −29° behaves similarly since the cosine is symmetric); higher toward Polaris, where the sky moves slowest and exposures can run longer. Around the pole star, trails become short arcs regardless.
- Default or precise?
- Default is the published NPF result and looks clean at normal viewing sizes. Precise halves it for large prints inspected up close.