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Rule of 500 Calculator

Use the Rule of 500 to get the longest shutter speed that keeps stars sharp, from focal length and crop factor.

Rule of 500

Enter the lens details. Results update live.

Max shutter speed
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Enter the focal length to get the shutter speed.

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What the Rule of 500 does

Stars trail because the Earth rotates, and longer lenses magnify that motion. Dividing 500 by the effective focal length gives the longest exposure before trails become visible. A 24mm lens on APS-C (1.5x crop) allows about 14 seconds.

Rule of 500 versus 300

The 500 rule is generous and suits web sized images, while the stricter 300 rule keeps stars pin sharp in large prints. High resolution sensors reveal trails sooner, so treat the 500 result as a maximum, not a target.

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Rule of 500 Calc FAQs

What camera settings for the Milky Way?
Use the widest aperture, the Rule of 500 shutter speed from the calculator, and ISO 1600 to 6400. Focus manually on a bright star with live view zoomed in.
Why are my stars blurry, not trailed?
Blur without trails usually means missed focus or a shaky tripod. Zoom live view to 10x on a bright star and focus until it is smallest; hang weight from the tripod in wind.
Do I need a star tracker?
Not to start. The Rule of 500 gives sharp untracked shots with wide lenses. Trackers help beyond 30 seconds or with telephoto lenses for deep sky objects.
What is the NPF rule?
A more precise formula accounting for aperture and pixel size. It usually gives shorter times than the 500 rule, so use it when the 500 result shows slight trailing.
Can I use the Rule of 500 on a phone?
Yes for the math, but most phones cap exposures at a few seconds. Use night mode stacking instead, or a manual camera app if it allows longer exposures.
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