The 500 rule was made for film. The NPF rule accounts for pixel size and aperture, so it gives realistic maximum shutter speeds for sharp, round stars on modern sensors.
Choose your camera from the list to load its crop factor and pixel pitch automatically.
The formulas
NPF (simplified): t = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel pitch in µm and f the focal length in mm. Strict NPF (Frédéric Michaud): t = (16.856N + 0.0997f + 13.713p) ÷ (f × cos δ) — stricter, and longer toward the celestial poles. 500 rule: t = 500 ÷ (f × crop factor).
Tips
- Pixel pitch = sensor width ÷ horizontal pixels. A 24 MP full-frame sensor is about 5.9 µm; 45 MP about 4.4 µm.
- Point at the celestial pole and you can expose longer; stars near the celestial equator move fastest.
- Stack several shorter exposures to reduce noise instead of exceeding the NPF time.
- Check each camera's page in our camera database for a ready-made table.
FAQ
Why is the NPF time so much shorter than the 500 rule?
Modern sensors have small pixels, so even a little star movement spreads across several pixels. The 500 rule ignores pixel size.
Which should I use?
Use the simplified NPF for web and social sizes, and the strict value if you print large or crop heavily.
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