Why stars trail
Earth turns once a day, so the stars appear to move across the sky by 15 arcseconds every second near the celestial equator. On a fixed tripod, a long exposure turns each star into a short line. To keep them as sharp points, the exposure has to be short enough that the movement stays within about a pixel or two on the sensor, yet long enough to collect light from faint stars and the Milky Way.
The 500 rule
The classic rule of thumb:
maximum exposure (s) = 500 ÷ (focal length × crop factor)
It works on the full-frame equivalent focal length, so a crop sensor shortens the exposure. Some photographers use a stricter 300 rule for modern high-resolution cameras.
The NPF rule
The 500 rule ignores the aperture and the pixel size. The NPF rule, proposed by Frédéric Michaud of the Société astronomique de France, takes both into account. Its simplified form is:
t = (35 × N + 30 × P) ÷ F
where N is the f-number, P the pixel pitch in micrometers and F the actual focal length in millimeters. The calculator also divides every rule by cos(declination): stars near the celestial pole move less, so you can expose longer there.
Worked example
A 24 mm lens at f/2.8 on a full-frame camera with 6 µm pixels, aimed at the celestial equator:
| Rule | Maximum exposure |
|---|---|
| 500 rule | 20.8 s |
| 300 rule | 12.5 s |
| NPF rule | 11.6 s |
During the NPF exposure, the stars move about 2.9 arcminutes across the sky. Point the same camera at a declination of 60°, around the Big Dipper, and every limit doubles.
With an APS-C camera (crop 1.5) and a 16 mm lens with 3.9 µm pixels at f/2.8, the 500 rule gives 20.8 s but the NPF rule only about 13.4 s.
Finding your pixel pitch
Divide the sensor width in millimeters by the image width in pixels, then multiply by 1,000:
| Camera type | Typical pixel pitch |
|---|---|
| 24 MP full frame | about 6 µm |
| 45 MP full frame | about 4.4 µm |
| 24 MP APS-C | about 3.9 µm |
| 20 MP Micro Four Thirds | about 3.3 µm |
Tips
- Shoot wide open, at a high ISO such as 3200 to 6400, and use a 2-second self-timer or a remote to avoid shake.
- Focus manually on a bright star using live view magnification: autofocus rarely works on the night sky.
- Plan around the new moon and away from city lights; the Milky Way core is best seen from spring to early fall in the Northern Hemisphere.
- For longer exposures without trails, use a star tracker, or take many short frames and stack them in software.