Point a camera at the night sky for long enough and the stars become short arcs. The Earth turns once every 23 hours 56 minutes, which moves the sky by about 15 arcseconds every second. A wide-angle lens hides that movement for a while; a longer lens or a higher-resolution sensor shows it sooner. Two rules of thumb try to predict the limit: the old 500 rule and the newer NPF rule.
The 500 rule
The 500 rule says the longest exposure in seconds is 500 divided by the full-frame equivalent focal length:
t = 500 ÷ (focal length × crop factor)
With a 24 mm lens on full frame, 500 ÷ 24 ≈ 21 s. On a 1.5× APS-C camera the same lens gives 14 s. The rule comes from the film era, when the aim was to keep trails invisible on a standard print. It ignores two things that matter today: the resolution of the sensor and the aperture of the lens. Because of that, many photographers use the stricter 400 or 300 rule, which simply swap in a smaller number.
The NPF rule
The NPF rule was developed by Frédéric Michaud of the Société Astronomique de France and popularised by PhotoPills. Its simplified form is:
t = (35 × N + 30 × p) ÷ f
where N is the f-number, p is the pixel pitch in micrometres, and f is the real focal length in millimetres (not the equivalent). Instead of guessing what is visible on a print, it aims to keep each star within a small number of pixels at 100% view.
The pixel pitch is the width of one pixel. You can estimate it from the sensor size and megapixels: p = √(sensor area ÷ number of pixels). A 24 MP full-frame sensor has a pitch of about 6.0 µm; a 45 MP sensor about 4.4 µm; a 61 MP one 3.8 µm. Our calculator does this for you from the megapixel count.
The aperture term reflects the fact that a lens wide open renders stars as slightly larger blur spots, which hide a little movement; stopping down makes stars smaller and trails easier to see.
How different are they?
On a 24 MP full-frame camera at f/2.8, the NPF rule allows roughly half the time of the 500 rule at wide focal lengths. On higher-resolution cameras the gap grows: with a 61 MP sensor and a 20 mm f/1.8 lens, the NPF limit is 8.8 s, while the 500 rule still says 25 s.
| Camera (full frame) | Pixel pitch | NPF at 20 mm f/1.8 | 500 rule |
|---|---|---|---|
| 24 MP | 6.0 µm | 12 s | 25 s |
| 33 MP | 5.1 µm | 11 s | 25 s |
| 45 MP | 4.4 µm | 9.7 s | 25 s |
| 61 MP | 3.8 µm | 8.8 s | 25 s |
Which rule should you use?
Use the NPF rule when you intend to print, crop, or view images at full size — which, for most people who bother with astrophotography, is always. Use the 500 rule (or 400 rule) when the photo will mostly be seen small, for example on social media, and you would rather have a cleaner, longer exposure with very slight trails. Many photographers treat the NPF result as the starting point and the 500 result as the absolute maximum.
Pointing direction matters
Both rules assume the worst case: stars near the celestial equator, which move fastest. Stars near the celestial poles — Polaris in the northern hemisphere, the south celestial pole in the southern — trace much smaller circles and move more slowly across the sensor. The movement scales with the cosine of the star’s declination, so when you aim high towards the pole you can expose noticeably longer than the rules suggest. The Milky Way core, low in the south from mid-northern latitudes, is close to the worst case.
A workflow for sharp Milky Way photos
- Choose the darkest sky you can, with the Moon below the horizon, and check the Milky Way’s position with a planning app.
- Use a wide, fast lens — 14–24 mm at f/1.4–f/2.8 — on a sturdy tripod.
- Focus manually on a bright star using magnified live view until it is the smallest possible point. Tape the focus ring if your lens drifts.
- Set the shutter to the NPF time rounded down to the nearest setting, the aperture wide open (or one third of a stop closed for sharper corners), and ISO 1600–6400.
- Shoot RAW with a 2-second delay, and check a test frame at 100% for trails and focus.
- For the foreground, take a separate frame focused closer (see the hyperfocal guide), and blend later.
When you need more light
If the NPF time is too short for a clean image, you have three options. Stack 10–30 identical short exposures and average them in software, which reduces noise by the square root of the number of frames. Use a star tracker, a motorised mount that rotates the camera with the sky, allowing exposures of minutes. Or switch to a faster lens: every stop of aperture is a stop of light, and the NPF rule only shortens a little when you open up.
Work out the exact limits for your own camera and lenses with the 500 rule and NPF calculator.