What is the 500 Rule?

The 500 Rule is a simple formula for astrophotography: divide 500 by your effective focal length (focal length × crop factor) to get the maximum exposure time in seconds before stars begin to visibly trail. For example, 500 ÷ 24mm = 20.8 seconds on a full-frame camera.

What is the difference between the 400 and 600 rules?

The 400 Rule is more conservative (shorter exposure) and works better with high-resolution sensors where trails are more visible. The 600 Rule is more lenient and works for lower resolution sensors or small prints. The 500 Rule is the most commonly used middle ground.

Does crop factor affect star trails?

Yes. A crop sensor magnifies the image, making star trails more visible at the same focal length. You must multiply your focal length by the crop factor before applying the rule. A 24mm lens on APS-C (1.5x) has an effective focal length of 36mm.

Is the 500 Rule accurate for modern cameras?

The 500 Rule was developed for film and lower resolution digital sensors. Modern high-megapixel sensors (40MP+) often show slight trailing even within the 500 Rule limit. Consider using the NPF Rule for more precise results with modern cameras.

PhotoCalcs

500 Rule Calculator — Maximum Star Exposure Time

Find the maximum exposure time before stars begin to trail using the 500 Rule, 400 Rule, or 600 Rule.

Focal Length

Your lens focal length

Sensor / Crop Factor

Full frame = 1x, APS-C = 1.5x

How We Calculate This

The 500 Rule provides a quick estimate of the maximum exposure time before star trailing becomes visible:

Max exposure (seconds) = 500 ÷ (focal length × crop factor)

The rule assumes that stars near the celestial equator move the fastest across the frame. At wider focal lengths you can expose longer because the angular movement per pixel is smaller.

Variations include the 400 Rule (stricter, for high-resolution sensors) and the 600 Rule (more lenient, for lower resolution or smaller prints).

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.