The 500 Rule is a simple formula — 500 divided by your full-frame-equivalent focal length — that estimates the longest shutter speed you can use before stars start visibly trailing.
The 500 Rule is the quickest way to get a workable shutter speed for sharp stars without opening an app: divide 500 by the full-frame-equivalent focal length and shoot no longer than that. At 24mm equivalent it gives about 21 seconds. Its value is speed and simplicity in the dark with cold hands.
Its weakness is that it only knows focal length, ignoring aperture, sensor resolution and where in the sky the camera is aimed, all of which affect how fast a star smears across pixels. On a modern 45-megapixel body the rule is often too generous by a wide margin, leaving stars as short streaks rather than points when the file is viewed at full size. Many photographers treat the 500 Rule result as a ceiling to stay well under, not a number to shoot right at.
Set up with a 20mm lens on a full-frame camera, the 500 Rule gives 500 divided by 20, which is 25 seconds. On a 24-megapixel sensor, a 25-second frame at f/2.8 shows stars that look like points at normal viewing size and only slightly stretched at 100 percent, which is fine for most uses.
Put the same lens on a 61-megapixel body and that 25-second frame now shows each star as an obvious short dash, because the finer pixel pitch resolves the movement the rule ignores. Dropping to 13 seconds fixes it but costs a stop of light, which has to come back through a wider aperture or ISO 6400 instead of 3200. On an APS-C body the same lens needs 500 divided by 30, about 16 seconds, since the crop factor changes the equivalent focal length.
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