Star Trails Calculator
Maximum exposure time before stars start to trail, using the NPF rule.
Results
- NPF rule max exposure
- 11.6s
- 500 rule max exposure
- 20.8s
- Pixel pitch
- 6.0 µm
The more accurate limit, accounting for your exact aperture and pixel density. Stay at or under this to keep stars as points, not streaks.
The simpler, older rule of thumb. It ignores resolution and tends to overestimate how long you can expose on modern high-megapixel cameras.
The physical size of one pixel on your sensor, derived from its resolution and width. Smaller pixels are less forgiving of star movement.
How it works
The Earth turns roughly 15 degrees an hour, so any exposure long enough smears stars into short streaks. How long is too long depends on the focal length, the aperture, and crucially the size of your pixels, since finer pixels record the same movement as more visible motion.
The 500 rule, dividing 500 by the effective focal length, dates from film and ignores resolution entirely. It was fine when grain set the limit, but on a modern high-megapixel sensor it consistently allows too much time and produces stars that look like short dashes at 100%.
The NPF rule accounts for aperture and pixel pitch, and it is noticeably more conservative. It is worth trusting: it is far easier to stack several shorter frames than to rescue a single trailed one.
The formula
t = (35 × N + 30 × p) / f
N is the aperture, p the pixel pitch in micrometres, and f the focal length in mm. This simplified form assumes stars near the celestial equator, where apparent motion is fastest, so it errs safe.
Frequently asked questions
- Which rule should I actually use?
- NPF. The 500 rule survives because it is easy to do in your head, but it was calibrated for film grain and gives visibly trailed stars on current sensors.
- Where does pixel pitch come from?
- The calculator derives it from your sensor dimensions and megapixel count, which matches published figures closely. A 24MP full-frame sensor works out at about 6 micrometres.
- Can I expose longer than this?
- Only with a star tracker, which rotates the camera to follow the sky and removes the limit entirely. Otherwise, shoot multiple shorter frames and stack them to build up signal without trailing.
- Does this apply to stars near the pole?
- It is conservative there. Stars near Polaris move much less across the frame, so you can expose considerably longer. The formula assumes the worst case near the celestial equator.