NPF Rule & Astrophotography Calculator
This astrophotography calculator uses the accurate NPF rule — alongside the familiar 500 rule — to find the longest shutter speed before stars start to trail, for your camera, lens and part of the sky. A second tab plans star trail sequences.
Plan a sequence of exposures to stack into star trails — by how long you'll shoot, or by the arc you want.
How to Use the Astrophotography Calculator
Sharp stars
- Enter your camera: Sensor and resolution, which together set the pixel size
- Enter your lens focal length and the aperture you'll shoot at
- Choose the part of the sky you're framing, or leave it at 0° for the worst case
- Use the NPF time as your longest exposure, and raise ISO to suit
Star trails
- Enter your camera and lens
- Plan by time or by arc: How long you can shoot, or how long you want the trails to be
- Set the exposure per frame and the gap your intervalometer leaves between frames
- Read the frame count, then stack the frames with a lighten blend
How to Calculate the NPF Rule
The NPF rule, published by Frédéric Michaud of the Société Astronomique du Havre, gives the longest exposure before stars stop looking like points. It is named after the three things it depends on: the aperture (N), the pixel pitch (P) and the focal length (F).
The NPF formula
t = (16.856 × N + 0.0997 × f + 13.713 × p) / (f × cos δ)
t is the longest exposure in seconds, N the f-number, f the focal length in millimetres, p the pixel pitch in micrometres and δ the declination of the stars you're framing. A simplified version, (35 × N + 30 × p) / f, is often quoted; the full version used here is more accurate across focal lengths.
To find the pixel pitch, divide the sensor width by the number of pixels across it: a 24-megapixel full-frame sensor is about 6,000 pixels wide, so its pitch is 36 mm ÷ 6,000 = 6 µm.
Worked example: the Milky Way core at 20mm f/1.8
On a 24-megapixel full-frame camera, aiming at the Milky Way's core (declination −29°):
- NPF: (16.856 × 1.8 + 0.0997 × 20 + 13.713 × 6) / (20 × cos 29°) = 6.6 seconds
- 500 rule for comparison: 500 / 20 = 25 seconds — long enough to show trailing at 100%
NPF rule vs 500 rule chart
Longest exposure for full-frame cameras at f/2.8, pointing at the celestial equator (the worst case). Higher-resolution sensors have smaller pixels, so they show trailing sooner.
| Focal length | 500 rule | NPF, 24MP | NPF, 45MP | NPF, 61MP |
|---|---|---|---|---|
| 14mm | 35.7 s | 9.3 s | 7.8 s | 7.2 s |
| 16mm | 31.3 s | 8.2 s | 6.8 s | 6.3 s |
| 20mm | 25 s | 6.6 s | 5.5 s | 5 s |
| 24mm | 20.8 s | 5.5 s | 4.6 s | 4.2 s |
| 35mm | 14.3 s | 3.8 s | 3.2 s | 2.9 s |
| 50mm | 10 s | 2.7 s | 2.2 s | 2.1 s |
On an APS-C or Micro Four Thirds camera the 500 rule divides by the crop factor too; the NPF rule already accounts for sensor size through the pixel pitch.
Understanding Night Sky Photography
The sky is always moving. How much that motion shows depends on your lens, your sensor and where you point.
The Sky Turns 15° an Hour
The Earth rotates once relative to the stars every 23 hours 56 minutes, so the sky appears to turn about the celestial poles at roughly 15° an hour.
Longer lenses magnify that motion, so the safe exposure time falls as focal length rises.
The 500 Rule
Divide 500 by the full-frame equivalent focal length for a maximum exposure in seconds. Simple, but made for film: it allows trails you'll see on a modern high-resolution sensor.
Example: 24mm on full frame → about 21 seconds.
The NPF Rule
A more accurate formula that includes pixel pitch, aperture and declination. Smaller pixels show trailing sooner, so high-resolution cameras need shorter exposures.
NPF keeps stars as points even at 100% view.
Declination
The sky's equivalent of latitude. Stars near the celestial equator (0°) cross the frame fastest; stars near the poles (±90°) barely move.
- 0°: Orion's belt, the worst case
- −29°: the Milky Way's core
- +89°: Polaris
Pixel Pitch
The width of one pixel on the sensor, in micrometres. A 24MP full-frame sensor has about 6µm pixels; a 61MP one about 3.8µm. Smaller pixels resolve finer detail — including the start of star trailing.
Stacking Star Trails
Rather than one exposure lasting hours, shoot many shorter frames and combine them with a lighten blend. You control noise, avoid over-exposing the sky, and can drop frames spoiled by aircraft or car headlights.
Keep the gap between frames to about a second so the trails stay continuous.
Astrophotography and NPF Rule FAQs
What is the NPF rule?
A formula for the longest exposure before stars trail, devised by Frédéric Michaud of the Société Astronomique du Havre. Unlike the 500 rule it accounts for pixel size, aperture and where in the sky you are pointing, so stars stay points even when you view the image at 100%.
Is the 500 rule still useful?
As a quick field estimate for small prints and social media, yes. It was devised for film and low-resolution sensors; on a modern 24–60 megapixel camera it allows trailing you'll see when you zoom in. A 24mm f/2.8 lens on a 24-megapixel full-frame camera gives about 21 seconds by the 500 rule but only about 5.5 seconds by NPF when aimed at the celestial equator.
Why do stars near Polaris trail less?
The sky appears to rotate around the celestial poles. Stars near the pole trace small circles and move slowly across the frame; stars near the celestial equator move fastest. That's why the calculator asks where in the sky you're pointing.
How long do I need for circular star trails?
The sky turns 15° an hour, so a quarter circle takes about six hours and a full circle about 24 hours — which is why complete circles need the long nights near the poles in winter. Two hours gives a satisfying 30° arc.
What settings should I start with for the Milky Way?
A wide lens at its widest aperture (f/1.4–f/2.8), ISO 3200–6400 on a modern full-frame camera, and the NPF exposure time for your setup. Shoot RAW, focus manually on a bright star using live view magnification, and use a sturdy tripod.