Star Trails / 500 Rule Calculator
Find the longest shutter speed you can use before stars start to trail. Enter your focal length, sensor size, and megapixels to get results from both the classic 500 Rule and the more precise NPF Rule.
Recommended Milky Way Settings
Suggested starting points for Milky Way photography at on .
| Scenario | Aperture | ISO | Shutter |
|---|---|---|---|
Max Exposure at Common Focal Lengths
Comparing both rules on at .
| Focal Length | 500 Rule | NPF Rule | Difference |
|---|---|---|---|
Understanding Star Trail Prevention
The Earth spins at roughly 15 degrees per hour. You won't notice it standing outside, but your camera absolutely will.
In astrophotography, you typically want sharp, pinpoint stars rather than streaks. So the real question becomes: how many seconds can you keep that shutter open before the rotation shows up in your image?
The 500 Rule Explained
The 500 Rule is the quick-and-dirty method. Take the number 500, divide it by your effective focal length (your lens's focal length multiplied by your sensor's crop factor), and you get your max shutter speed in seconds.
So a 24mm lens on full frame? That's 500 / 24 = roughly 20 seconds. Simple enough.
Here's the catch. This rule comes from the film era, when sensors had far fewer pixels. It still works as a starting point, but if you're shooting with a modern 45MP or 60MP camera, you'll almost certainly see trailing before you hit that number. The pixels are just too small and too numerous to forgive the motion.
Why the NPF Rule Is More Accurate
The NPF Rule was developed by Frederic Michaud for the Societe Astronomique de France, and it accounts for three things the 500 Rule completely ignores: your sensor's pixel density (N), your lens aperture (P), and your focal length (F).
It works by calculating your pixel pitch (derived from sensor size and megapixel count), then figuring out exactly when a star's movement would span more than a single pixel. The result is almost always shorter than the 500 Rule suggests.
And that shorter number? It's the accurate one.
Star Photography Basics
Shutter speed is only part of the puzzle. You also need to nail your aperture and ISO. Open your lens as wide as it goes, ideally somewhere between f/1.4 and f/2.8. Set your ISO between 1600 and 6400 depending on how well your camera handles noise.
Always shoot RAW. The Milky Way's core will look like a faint smudge on the back of your camera, but there's a ton of detail hiding in that RAW file waiting to be pulled out in post.
Focusing in the dark is the hardest part. Switch to manual focus, zoom your live view to 10x on a bright star, and adjust until it's the tiniest possible point. Once you've got it, tape that focus ring down. One accidental bump and your entire session is soft.
Dealing with Light Pollution
Light pollution will ruin your shots faster than any wrong setting. Even moderate light pollution washes out the Milky Way and kills the number of visible stars.
Check Light Pollution Map (lightpollutionmap.info) to find dark sky spots near you. Plan on driving at least 30 to 60 minutes from any major city. The difference is dramatic.
A light pollution filter (didymium or narrowband) can help in moderately polluted areas by cutting sodium and mercury vapour wavelengths. But honestly? Nothing replaces genuinely dark skies. If you can get to a Bortle 3 or lower zone, do it.