What is the NPF Rule?

The NPF Rule was developed by Frédéric Michaud for the Société Astronomique de 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.

Why is the NPF Rule more accurate than the 500 Rule?

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.

What is declination and how does it affect star trails?

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.

What pixel pitch do I need?

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.

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.

Focal Length

Aperture (f-number)

Sensor

Sensor size and megapixel count

Declination

0° = equator, 90° = pole

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 accounts for the Airy disc diameter (35 × N) and pixel size (30 × p) to ensure trails are shorter than 2 pixels. Stars closer to the celestial equator (declination = 0°) trail faster, hence the cos(dec) divisor.

Frequently Asked Questions

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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.