NPF rule

Definition

The NPF rule (Nénette / Pixel / Focal) estimates max untracked seconds as accuracy × (35N + 30p) / f, with N the f-number, p pixel pitch in µm and f the focal length in mm. Accuracy 1 is pixel-sharp; 2 allows a little trailing. It beats the 500 rule on dense modern sensors. Declination, tracking error and aggressive sharpening still move the goalposts. Use it as a starting shutter, then inspect a 100 % crop of a test star.

Calculators that use this

• Astrophotography

  NPF Rule Calculator

  Get a pixel-aware untracked star shutter from focal length, aperture, pixel pitch and an accuracy factor — tighter than the 500 rule on high-MP bodies.

• Astrophotography

  500 Rule Calculator

  Estimate the longest untracked shutter for stars with the 500, 300 and 200 rules from focal length and crop factor.

• Astrophotography

  Milky Way Exposure Calculator

  Starting Milky Way shutter from the 500 rule, a suggested ISO scaled from f/2.8 and 20 s at 3200, plus night EV at those settings.