If you’re new to astrophotography and want to capture stunning images of the night sky, you might have heard of the 500 Rule—a simple formula to help you take long-exposure photos of stars without them turning into streaks due to Earth’s rotation. But did you know there are also the 400 Rule and 300 Rule?
In this post, we’ll break down these rules, explain why they matter, and help you decide which one to use for your astrophotography setup.
What Is the 500 Rule?
The 500 Rule is a quick and easy way to estimate the longest exposure time you can use without getting noticeable star trails in an image taken with a camera on a fixed tripod. The formula is:
Shutter Speed (seconds)=500 ÷ Focal Length of Your Lens
For example, if you’re using a 24mm lens on a full-frame camera, your max exposure time would be:
500 ÷ 24=20.8 seconds
So, you can expose for about 20 seconds before star trails become obvious.
What About Crop Sensor Cameras?
If you’re using a crop sensor (APS-C or Micro Four Thirds), you need to factor in the crop factor of your camera. The formula then becomes:
500 ÷ (Focal Length×Crop Factor)\
For example, if you have an APS-C camera with a 1.5x crop factor and a 24mm lens:
500 ÷ (24×1.5) = 13.9 seconds
So, you’d want to limit your exposure to 13 seconds instead of 20.
Why Use the 400 or 300 Rule?
Modern cameras have high-resolution sensors that capture more detail than older cameras. The problem? They also reveal small amounts of star trailing that might not have been noticeable before. That’s where the 400 Rule and 300 Rule come in.
500 vs. 400 vs. 300 Rule Comparison
| Rule | Formula (Shutter Speed in Seconds) | Best For |
|---|---|---|
| 500 Rule | 500 ÷ Focal Length | Older cameras (12-24 MP) or casual wide-field shots. |
| 400 Rule | 400 ÷ Focal Length | Mid-range cameras (24-36 MP) with better sharpness. |
| 300 Rule | 300 ÷ Focal Length | High-resolution cameras (40+ MP) or large prints. |
Example with a 24mm Lens on a Full-Frame Camera
| Rule | Max Exposure Time |
|---|---|
| 500 Rule | 20.8s |
| 400 Rule | 16.7s |
| 300 Rule | 12.5s |
The lower the number, the shorter the exposure, reducing the chance of star trailing.
Which Rule Should You Use?
- If you’re just starting out and don’t plan on printing large images, the 500 Rule is a great place to begin.
- If you want sharper images, especially with a mid-range resolution camera (24-36 MP), use the 400 Rule.
- If you have a high-resolution camera (40+ MP) or plan to print large images, use the 300 Rule for maximum sharpness.
That said, these are just guidelines. The best way to find what works for you is to experiment with different exposure times and zoom in on your images to check for star trailing.
Beyond the 500 Rule: Tracking Mounts
If you want even longer exposures without star trails, consider using a tracking mount. These devices move your camera at the same rate as Earth’s rotation, allowing you to take minutes-long exposures that reveal fainter stars, nebulae, and galaxies.
For those ready to take their astrophotography to the next level, a star tracker is one of the best investments you can make!
Final Thoughts
The 500, 400, and 300 Rules are great tools for beginner astrophotographers who want to take sharp images of the night sky without complicated equipment. Start with the 500 Rule, experiment with shorter exposure times, and see what works best for your camera and lens.
And remember—rules are meant to be guidelines, not strict laws. The night sky is your canvas, so get out there and start shooting!
Happy stargazing!