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Stargazing Secrets: The 500 Rule Guide
Capturing the Milky Way over Ribblehead Viaduct or the stars above Malham Tarn is a bucket-list experience for many photographers. However, because the Earth is constantly rotating, the stars actually move across your frame. If your shutter is open for too long, those tiny points of light turn into blurry “sausages” or streaks known as star trails.
What is the 500 Rule?
The 500 Rule is a simple mathematical shortcut used to calculate the maximum number of seconds you can leave your shutter open before the movement of the Earth becomes visible in your photo.
- Pinpoint Stars: Keep the stars looking like sharp, crisp dots.
- Consistency: No more “trial and error” in the dark and cold.
- Focal Length: Understand why wider lenses allow for much longer exposures.
Understanding the Maths
The rule is based on your “effective” focal length. Because smaller sensors “crop” the image, they effectively magnify the movement of the stars, meaning you need a faster shutter speed than a full-frame camera would.
Max Seconds = 500 / (Focal Length × Crop Factor)
For example, if you are using a 24mm lens on a standard Nikon APS-C camera (1.5x crop), the maths looks like this:
500 / (24 × 1.5) = 500 / 36 = 13.8 seconds (Round down to 13s)
Common Crop Factors
| Sensor Type | Common Cameras | Factor |
|---|---|---|
| Full Frame | Nikon Z8, Canon R5, Sony A7IV | 1.0x |
| APS-C (Nikon/Sony/Fuji) | Nikon Z50, Sony A6400, Fuji XT-5 | 1.5x |
| APS-C (Canon) | Canon R7, R10, 90D | 1.6x |
| Micro Four Thirds | OM System OM-1, Panasonic GH6 | 2.0x |
Top Tips for Sharp Stars
- Focus on Infinity: Use ‘Live View’ to zoom in on a bright star and manually focus until it is as small as possible.
- Wide Aperture: Use your lowest f-number (e.g., f/2.8 or f/1.4) to let in as much light as possible during your short window.
- The 2-Second Timer: Even pressing the shutter button causes vibration. Use a 2-second delay so the camera is perfectly still when the exposure begins.
Ready to chase the Milky Way? Our Night Sky & Milky Way Workshops take place in the darkest corners of North Yorkshire. View our upcoming Astro dates here.