Astrophotography Made Simple
If you’ve ever tried astrophotography—or wanted to—you’ve probably run into the challenge of avoiding star trails. The same applies if you’re photographing the Northern Lights. Those little streaks in your photo happen when stars move across your frame during a long exposure, making them look blurry or out of focus.
Thankfully, there’s a simple rule that helps keep your stars sharp:
The Astrophotography Rule
Take the focal length of your lens (in millimeters) and divide it into either 500 or 600.
The result gives you the maximum number of seconds you can expose your shot before stars start to trail.
Example:
- Using a 24mm lens → 600 ÷ 24 = 25 seconds max exposure.
500 vs. 600?
For years, I’ve used the 600 rule and been very happy with the results. Recently, though, I’ve been testing under extreme magnification and noticed very slight movement at times. That’s led me to experiment with the 500 rule for sharper results.
So far, the difference isn’t dramatic, try both and see which works best for your photography style.
All Things Equal – Astrophotography
This rule assumes two important things:
- You’re working with a steady tripod.
- Your ISO is set properly for night shooting.
With that in place, this guideline gives you a solid starting point for finding the balance between tack-sharp stars and the beginnings of star movement.
One key detail: always calculate using the 35mm equivalent focal length of your lens.
How It Works
Full-Frame Cameras
On a full-frame sensor, here’s how you apply the rule:
- 50mm lens → 600 ÷ 50 = 12 seconds
You can expose for up to 12 seconds before star trails appear. - 24mm lens → 600 ÷ 24 = 25 seconds
You can expose for up to 25 seconds before star trails appear.
Astrophotography with APS-C Sensors
If your camera uses an APS-C sensor, you’ll need to factor in the crop conversion ratio (typically 1.6). That means you first multiply your lens focal length by 1.6 before applying the rule.
Example with a 50mm lens:
- 50 × 1.6 = 80mm equivalent
- 600 ÷ 80 = 7.5 seconds
→ You can safely expose for about 8 seconds before star trails appear.
Example with a 24mm lens:
- 24 × 1.6 = 38.4mm equivalent
- 600 ÷ 38.4 = 15 seconds
→ You can expose for up to 15 seconds before star trails show.
If you want to learn more you can take one of my courses I teach online.
