PhotoCalcs
NPF Rule Calculator — Sharp Stars Exposure
Calculate the precise maximum exposure time for sharp stars using the NPF Rule, which accounts for pixel pitch, aperture and declination.
How We Calculate This
The NPF Rule formula calculates the maximum exposure time for pinpoint stars:
t = (35 × N + 30 × p) / (f × cos(dec))
Where N = aperture f-number, p = pixel pitch in micrometres, f = focal length in mm, and dec = declination of the area of sky being photographed.
The formula combines the aperture-driven Airy disc diameter (35 × N) with pixel size (30 × p). These simplified coefficients match the “default” tolerance used by PhotoPills’ Spot Stars — trailing that is barely noticeable (roughly one to two pixels) at normal viewing sizes. For pin-sharp stars in very large prints, the stricter “accurate” setting roughly halves these exposure times. Stars closer to the celestial equator (declination = 0°) trail faster, hence the cos(dec) divisor.
Frequently Asked Questions
The NPF Rule was developed by Frédéric Michaud of the Société Astronomique du Havre (the Astronomical Society of Le Havre, France). It calculates the maximum exposure time based on aperture (N), pixel pitch (P), and focal length (F), giving much more accurate results than the 500 Rule for modern high-resolution sensors.
The 500 Rule only considers focal length and was designed for film. The NPF Rule accounts for pixel pitch (sensor resolution), aperture (which affects the Airy disc diameter), and star declination, producing tighter and more accurate limits for modern digital cameras.
Declination is a star's angular distance from the celestial equator. Stars at the equator (0°) move fastest across the sky and trail the most. Stars near the poles (±90°) move very slowly. Photographing towards Polaris allows longer exposures.
Pixel pitch is calculated from your sensor size and megapixel count. Higher resolution sensors have smaller pixel pitches and are more sensitive to star trailing. A 24MP full-frame sensor has ~6µm pixels, while a 61MP full-frame has ~3.7µm pixels.
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Last updated: February 2026
All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.