500 Rule & NPF Calculator
Star-sharp exposure time for astrophotography
Camera & Lens
Stars near the pole move less, so you can expose longer
Longest exposure without star trails (NPF rule)
22.7 s
500 rule (lower-resolution cameras)35.7 s
300 rule (high-resolution cameras)21.4 s
Pixel pitch6.00 µm
Full-frame equivalent14mm
The 500 rule dates from film and low-resolution sensors; at 100% view on modern cameras stars look elongated. Use the NPF value for pixel-sharp stars.
Max Shutter Speed by Focal Length
| Lens | 500 rule | 300 rule | NPF |
|---|
Starting Settings
| Situation | Aperture | ISO | Shutter |
|---|---|---|---|
| Milky Way, dark site | f/1.8~2.8 | 3200~6400 | NPF value |
| Starry landscape, moonlight | f/2.8~4 | 1600~3200 | 10–20 s |
| Star trails (stacked) | f/2.8~4 | 400~1600 | 20–30 s × many frames |
Milky Way Season
| Months | Milky Way core (Northern Hemisphere) |
|---|---|
| Feb–Apr | Briefly before dawn in the east |
| May–Aug | Most of the night in the south — best season |
| Sep–Oct | Early evening in the southwest |
| Nov–Jan | Mostly hidden (behind the Sun) |
Plan around the new moon and dark skies — check the moon phase calculator.
Formulas & Sources
- 500 rule: 500 ÷ (focal length × crop factor) · 300 rule: 300 ÷ (focal length × crop factor)
- NPF rule (Frédéric Michaud, simplified): (35 × f-number + 30 × pixel pitch µm) ÷ focal length, divided by cos(declination)
- Pixel pitch = sensor width ÷ horizontal pixels · suggested shutter = nearest 1/3-stop value at or below the result
Last reviewed: 2026-10-19
FAQ
What is the 500 rule?
Divide 500 by (focal length × crop factor) to get the longest exposure before stars trail. A 14 mm lens on full frame gives about 35 s; on 1.5× APS-C about 24 s.
How is the NPF rule different?
NPF also accounts for pixel size and aperture, so stars stay pinpoint on high-resolution sensors. A 24 MP full-frame camera at 14 mm f/2.8 aimed at the Milky Way core gets about 23 s.
When is Milky Way season?
In the Northern Hemisphere the core is best from May to August, visible before dawn from February and in the early evening into October. Shoot near the new moon.
What ISO should I use for the Milky Way?
Start around ISO 3200–6400 at f/2.8 from a dark site. Stack several frames to reduce noise.
Why can I expose longer near Polaris?
Stars circle the celestial pole, so those near it move less across the sensor. Pick a declination to scale the exposure.