Star Trails Rule 500

The 500 rule for star photography

The 500 rule estimates the longest shutter speed (in seconds) that still records stars as sharp dots, before Earth’s rotation drags them into trails: t_max = 500 / focal_length_FF. A 24 mm full-frame lens gives you 500 / 24 ≈ 20 s; a 50 mm cuts that to 10 s. On crop sensors, plug in the 35 mm equivalent focal length instead (multiply by 1.5× for APS-C, 2× for Micro Four Thirds).

Keep in mind it’s a rough estimate carried over from the film era. Modern high-resolution sensors (24 MP+) show trailing sooner, which is why a lot of astrophotographers reach for the stricter NPF rule, which accounts for pixel pitch, aperture and declination. Want round stars on long exposures no matter the focal length? Mount an equatorial tracker like the Sky-Watcher Star Adventurer or iOptron SkyGuider Pro. It turns the camera at sidereal rate and cancels out the star motion.

Applications

Wide-field Milky Way landscapes, meteor-shower captures, deep-sky imaging with star trackers, and the reverse case too: deliberate star trails built by stacking many long exposures in StarStaX or Sequator. The 500 rule helps you pick the shutter for the look you’re after. Stay below it and stars stay pinpoints; go above and they streak.

FAQ

Does the rule work for any sensor? Not exactly. It was calibrated for 35 mm film, so on a 45 MP sensor you might already spot trailing at 70 % of the calculated time. Check at 100 % zoom.

What is the difference vs. the 600 or 400 rule? Same idea, different tolerance. The 400 is stricter and leaves less trail, the 600 is more permissive, and the 500 sits in the popular middle.

Do I need an equatorial mount for Milky Way shots? Not at all. Untracked exposures of 15–25 s with wide lenses (14–24 mm) at f/1.8–2.8 and ISO 3200–6400 give excellent results. A tracker only becomes essential once you push past ~50 mm or chase deep-sky targets.