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.

What is the NPF Rule Star Trail Calculator?

Enter aperture, pixel pitch, focal length, and declination to compute the maximum exposure time before stars trail, using the accurate NPF rule. A far better limit than the old 500 rule for sharp Milky Way photos. It runs free in your browser on Gera Tools, with nothing uploaded.

NPF Rule Star Trail Calculator

Name: NPF Rule Star Trail Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

Why the 500 rule fails on modern cameras

The 500 rule says: maximum shutter speed in seconds = 500 ÷ focal length. On a 24mm lens, that gives about 21 seconds. On a 35mm full-frame sensor with 24 megapixels (pixel pitch ~5.9 µm), 21 seconds may be fine. But modern cameras are denser. A 45-megapixel full-frame sensor has a pixel pitch around 4.3 µm, and a 61-megapixel camera drops below 4 µm. At that resolution, stars trail noticeably within 10–12 seconds on a 24mm lens. The 500 rule was calibrated for older, lower-resolution film and digital sensors.

The NPF rule solves this by including both aperture and pixel pitch in the formula.

How the NPF formula works

The base NPF exposure limit, in seconds:

t = (35 × N + 30 × p + 250 × c) / f

where:

N = aperture f-number
p = pixel pitch in microns (sensor width in mm ÷ horizontal pixel count × 1000)
c = a celestial constant (set to a fixed value in the simple form)
f = focal length in millimetres

The accurate form adds a declination correction, because stars near the celestial equator sweep faster than stars near the poles:

t_accurate = t / cos(declination)

Pointing at the Milky Way core (near the galactic centre at roughly −28° declination), the cosine correction is modest. Pointing near the celestial equator gives the shortest allowed exposure; pointing near Polaris allows a much longer one.

What each term does:

35 × N — larger aperture = less depth-of-field depth for the point spread function; limits exposure as f-number grows.
30 × p — finer pixels resolve more motion; limits exposure as pixel pitch shrinks.
250 × c / f — focal length stretches the apparent motion across more pixels; limits exposure as focal length grows.

Worked example

A photographer uses a 24mm lens at f/2.8 on a camera with a 4.3 µm pixel pitch, pointing toward the Milky Way core at declination −28°:

Base NPF limit: (35 × 2.8 + 30 × 4.3 + 250 × 1) / 24 ≈ (98 + 129 + 250) / 24 ≈ 19.9 seconds
Declination correction: 19.9 / cos(−28°) ≈ 19.9 / 0.883 ≈ 22.5 seconds (stars near the equator = shorter; south celestial hemisphere corrects upward here)

Compare this with the 500 rule: 500 / 24 ≈ 21 seconds — similar here, but on a higher-resolution sensor the NPF limit would drop much further below the 500 rule estimate.

Finding your pixel pitch

Pixel pitch is not always listed in camera specs, but it is easy to calculate:

pixel pitch (µm) = sensor width (mm) / horizontal pixel count × 1000

For example, a common full-frame sensor that is 36 mm wide with 6720 horizontal pixels: 36 / 6720 × 1000 = 5.36 µm

Shooting strategy when NPF limits are short

If your NPF limit comes out at 8–10 seconds — common on high-resolution bodies with telephoto lenses — a single-exposure approach loses most of your total light. Instead:

Take many short exposures (20–100 frames) at the NPF limit.
Stack them in software (Sequator, StarrySkyStacker, Siril, or Starry Landscape Stacker for foreground blending).
Stacking averages out noise the same way a long exposure would, without trailing.

This workflow is also more resilient to passing clouds or satellite trails. Each bad frame can be excluded from the stack. The NPF calculator helps you know exactly how short each sub-exposure needs to be before you shoot.