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Astrophotography 500 Rule Calculator

Find the longest shutter time before stars trail on an untracked shot using the 500 rule, adjusted for your crop factor and a stricter 400 or 300 variant.

Focal length

Sensor / crop factor

Rule variant

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How it works

Max seconds = rule number (500 / 400 / 300) ÷ (focal length × crop factor)

The Earth’s rotation makes the stars drift across the frame, so on a fixed tripod there is a limit to how long you can expose before pinpoint stars smear into short trails. The 500 rule estimates that limit: divide 500 by your full-frame-equivalent focal length (actual focal length times crop factor) to get the maximum shutter time in seconds. It is deliberately lenient — modern high-resolution sensors resolve trailing sooner, so many astrophotographers switch to the 400 or even 300 rule for critically sharp stars when viewed at 100%. To expose longer than the rule allows, you need a motorised star tracker that turns with the sky.

Worked example

A 24 mm lens on a full-frame body (crop 1.0×) under the 500 rule allows 500 ÷ 24 ≈ 20.8 seconds. Put the same lens on an APS-C body (1.5×) and the effective focal length becomes 36 mm, cutting the limit to 500 ÷ 36 ≈ 13.9 seconds.

Frequently asked questions

What is the 500 rule?

A quick guideline for untracked night-sky shots: the longest exposure in seconds before stars trail is about 500 divided by your effective (full-frame-equivalent) focal length. It trades a little sharpness for a simple number you can work out in your head at the tripod.

Should I use the 500, 400 or 300 rule?

The 500 rule is lenient and fine for web-sized images or wider shots. Use the 400 or 300 rule for high-megapixel sensors or when you plan to view at 100% or print large, since those cameras reveal star trailing that the 500 rule would let through.

How do I expose longer than the rule allows?

Use a star tracker (an equatorial mount that rotates the camera with the sky), which cancels the Earth’s motion and lets you expose for minutes without trailing. Otherwise, raise ISO or open the aperture to gather more light within the time the rule permits.

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This calculator is for educational purposes. Double-check important results before acting on them.