NPF Rule
The NPF rule computes the maximum shutter speed before star trailing becomes visible, using a formula that factors in aperture (N), pixel pitch (P) and focal length (f): t = (35N + 30p) / f, with pixel pitch in microns and focal length in millimeters. Unlike the simpler 500 rule, it accounts for your specific camera's resolution, and its full form additionally corrects for the target's declination.
A smaller pixel pitch (higher resolution) shortens the computed safe shutter speed, since the identical physical star motion becomes visible sooner on more tightly packed pixels. This is exactly the gap the 500 rule, which has no resolution term at all, can't account for — the NPF rule's answer diverges further from the 500 rule's as camera resolution climbs.
Frequently Asked Questions
Do I need to know my camera's pixel pitch to use the NPF rule?
No — this site's calculator derives it automatically from your sensor format and megapixel count, both easy-to-find spec-sheet numbers, so you never need to calculate or look up pixel pitch yourself.
Is the NPF rule's coefficient set (35 and 30) universally agreed on?
It's a widely published, commonly used approximation, but different sources do publish slightly different coefficients — worth knowing before comparing this site's figure against another calculator's for the identical camera and settings.