500 Rule Calculator

Find the longest exposure before stars trail, using the 500 rule (and the stricter 300 and 200 variants) with your focal length and sensor crop factor. Calculated in your browser.

Max shutter before star trails

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Effective focal length (35mm)-

Show the math

The 500 rule estimates the longest exposure before the Earth's rotation turns stars into streaks:

max seconds = rule ÷ (focal × crop factor)

Use the effective (35mm-equivalent) focal length, a crop sensor magnifies movement, so it needs a shorter exposure. The stricter 300 and 200 rules suit high-resolution sensors and tighter inspection.

Crop factor is taken from the selected sensor. All math runs in your browser.

What this does

A 500 rule calculator finds the longest exposure before stars trail, using the 500 rule and the stricter 300 and 200 variants with your focal length and sensor crop factor.

How to use it

  1. Enter the focal length.
  2. Enter the crop factor.
  3. Read the 500-rule limit.
  4. Compare the 300 and 200 limits.

How it works

Use the 35 mm-equivalent focal length, so crop sensors get a shorter time. High-resolution sensors reveal trailing sooner, so the 300 or 200 divisor is safer.

max seconds = 500 ÷ (focal length × crop factor)

Understanding your result

This is the longest exposure before stars visibly trail, and the stricter 300 and 200 variants exist because the original 500 rule was set for lower-resolution film; a modern high-megapixel sensor reveals trailing sooner, so use the smaller divisor if you pixel-peep. Always use the 35mm-equivalent focal length, which is why a crop sensor gets a shorter time than the bare focal length suggests.

Example

A 20 mm lens on full frame allows about a 25-second exposure under the 500 rule.

Sources & methodology

Last updated .

Frequently asked questions

How does the 500 rule work?

It estimates the maximum exposure before stars streak: max seconds = 500 ÷ (focal length × crop factor). Use the 35mm-equivalent focal length so crop sensors get a shorter time.

When should I use the 300 or 200 rule?

High-resolution sensors and close inspection reveal trailing sooner, so the stricter 300 and 200 divisors give a safer, shorter exposure.

Is anything uploaded?

No. The calculation runs entirely in your browser — nothing you enter is sent anywhere.