500 Rule Calculator

A rule of thumb for the longest shutter speed before stars visibly trail from Earth's rotation: 500 divided by the full-frame-equivalent focal length. A 20 mm lens on full frame allows about 25 seconds; on APS-C the same lens allows about 16 seconds.

Earth rotates 15 degrees per hour, so stars drift across the frame during the exposure. Longer focal lengths magnify the drift, which is why wide lenses allow much longer exposures before trailing shows.

The 500 rule dates from film and modest print sizes. Modern high-resolution sensors reveal trailing sooner, so many shooters use 400 or the conservative 300 rule. If you pixel-peep or print large, choose 300; the calculator shows all three.

A more rigorous formula that also accounts for aperture and pixel pitch, typically giving shorter, sharper limits than the 500 rule. The 300 rule shown here approximates NPF results for common setups without needing your exact pixel pitch.

A wide, fast lens (14–24 mm at f/1.4–f/2.8), the shutter time from this calculator, ISO 1600–6400, manual focus on a bright star with magnified live view, and a solid tripod with a 2-second timer or remote.

Yes — the rule uses the full-frame-equivalent focal length, so a 20 mm lens on a 1.5× APS-C body counts as 30 mm and allows a shorter exposure. The calculator applies your sensor's crop automatically.

A star tracker rotates the camera with the sky, allowing minutes-long exposures at low ISO. Alternatively, stack many shorter frames in software to reduce noise without trailing.

Yes — shutter speed, ISO, aperture and focal length are stored in the photo's EXIF metadata. Read them with the free EXIF viewer on this site, or the EXIF Viewer by Fluntro app on iPhone and iPad.

No — the calculation runs entirely in your browser with no network requests, and the page works offline once loaded: useful under a dark sky with no signal.

Yes — completely free, no account. For reading the metadata of your night shots on iPhone and iPad, the EXIF Viewer by Fluntro app is a one-time purchase.