Rule of 500 — Star Trail Calculator
Calculate the maximum shutter speed to avoid star trailing in astrophotography using the Rule of 500, 600, and 300.
Results
What is it?
Stars appear to trail across the sensor due to Earth's rotation. The Rule of 500 is a quick rule of thumb: divide 500 by your effective (equivalent) focal length to get the maximum shutter speed in seconds before stars become visible trails at normal print/screen sizes. The Rule of 300 is more conservative for high-megapixel cameras; Rule of 600 is more permissive.
How to use
Enter the physical focal length of your lens and select your sensor crop factor. The three rule results appear simultaneously. Use the strictest (Rule of 300) for sensors above 24 MP, the standard (Rule of 500) for typical use, and Rule of 600 only for very short focal lengths or low-resolution sensors.
Example scenario
A 24 mm lens on a full-frame camera: Rule of 500 gives 20.8 s, Rule of 300 gives 12.5 s. The same lens on an APS-C body (1.5×): effective FL = 36 mm, so Rule of 500 = 13.9 s. Wide lenses always allow longer exposures because the apparent angular motion of stars covers fewer pixels.
Pro tip
For truly pinpoint stars on modern high-resolution sensors (45 MP+), astrophotographers use the NPF rule: (35 × aperture + 30 × pixel pitch) / focal length. Alternatively, use a star tracker to allow 2–5 minute exposures for superior deep-sky images without any trailing at all.