Why is the NPF rule better than the 500 rule?

The 500 rule only divides 500 by focal length and ignores sensor resolution and aperture. The NPF rule accounts for pixel pitch and aperture, so it gives a much shorter, more realistic limit on modern high-megapixel cameras where the 500 rule leaves visible trails.

What is pixel pitch and how do I find it?

Pixel pitch is the physical spacing between pixels on your sensor, in microns. Divide your sensor width in millimetres by the horizontal pixel count, then multiply by 1000. A 36mm full-frame sensor with 6000 pixels has a 6 micron pitch.

Why does declination matter?

Stars near the celestial equator (declination 0) sweep across the sky fastest, while stars near the poles barely move. The NPF rule divides by the cosine of declination, so pointing toward the equator gives the shortest allowed exposure.

What are the constants 35, 30, and 250?

They are empirical coefficients in the NPF formula by Frederic Michaud. The 35 weights aperture, the 30 weights pixel pitch, and the 250 scales the focal-length term so the result comes out in seconds for typical sensors and lenses.

Should I use the simple or the accurate NPF result?

The accurate result divides by the cosine of declination for the star field you are framing and is the strictest. The simple result assumes the worst case at the equator. Use the accurate one when you know where you are pointing.

NPF Rule Star Trail Calculator

The classic 500 rule for night photography is too generous for today’s high-resolution sensors and leaves stars as visible streaks. The NPF rule, developed by Frederic Michaud, factors in aperture and pixel pitch to give a much more realistic maximum exposure time before star trailing becomes visible.

How it works

The base NPF time, in seconds, combines three terms:

t = (35 × N + 30 × p + 250 × c) / f

where N is the aperture f-number, p is the pixel pitch in microns, c is a celestial term, and f is the focal length in millimetres. The simple form sets c to a constant. The accurate form additionally divides the final time by the cosine of the target declination, because stars near the equator move faster:

t_accurate = t / cos(declination)

A larger aperture, finer pixel pitch, or longer focal length all shorten the allowed exposure, exactly as you would expect for keeping point-like stars.

Example and tips

A 24mm lens at f/2.8 on a sensor with a 4.3 micron pixel pitch gives a base NPF limit of roughly 10 seconds, versus about 21 seconds from the old 500 rule — that difference is the streaking the 500 rule hides. If you point near the celestial equator, expect the accurate limit to be the shortest. When you need longer total integration, take many short NPF-compliant frames and stack them rather than risking a single trailed exposure.