How long can you expose the night sky before the stars trail?
Enter your sensor, lens and aperture to see the longest sharp exposure from the 500 rule, the 300 rule and the NPF rule. Then plan how many frames you need for a star trail.
How the tool works
The 500 rule divides 500 by your focal length multiplied by the crop factor. A 14 mm lens on full frame gives 500 / 14 = 35.7 seconds. It was written for film and low resolution sensors, so on modern high resolution cameras it often gives stars that are slightly stretched at full size. The 300 rule uses 300 instead and is a safer starting point.
The NPF rule also takes the f-number and the pixel pitch into account: (35 x f-number + 30 x pixel pitch in micrometres) / focal length. A 24 megapixel full frame sensor has a pixel pitch of about 6 micrometres, so at 14 mm and f/2.8 the NPF time is (35 x 2.8 + 30 x 6.0) / 14, or about 19.9 seconds. If you know your exact pixel pitch, type it in. Otherwise the calculator estimates it from the sensor width and the megapixels.
Stars appear to move fastest at the celestial equator and slowest near Polaris, so the allowed time is divided by the cosine of the declination. From the UK, looking south at around 38 degrees up is close to the equator, and north is near Polaris. The star trail planner uses 15 degrees of sky rotation per hour to work out the time, frames and card space. All of these are rules of thumb, so take a test shot and zoom in to check.
Questions
What is the 500 rule in astrophotography?
It estimates the longest exposure in seconds before stars start to trail: 500 divided by the focal length, after multiplying by the crop factor. It is a rough guide and tends to be too long for high resolution sensors.
Is the NPF rule better than the 500 rule?
It is usually more accurate on modern cameras because it uses the aperture and pixel size as well as the focal length. It is still an estimate, and the result depends on how closely you look at the image.
How many frames do I need for a star trail?
Divide the total time by the exposure plus the gap between frames. One hour at 30 seconds with a 1 second gap is about 117 frames. Stack them in software to join the trails.
More photography tools: Focal Length Field of View Calculator · Moon Phase and Milky Way Planner