Rule of 500
Rule of 500 (Astrophotography)ExposureMaximum untracked exposure time before stars trail: 500 ÷ (crop factor × focal length).
Maximum untracked exposure time before stars trail: 500 ÷ (crop factor × focal length). For a 16mm lens on APS-C (crop 1.5): 500 ÷ (1.5 × 16) = 20 seconds. A stricter version using 300 is preferred for high-resolution sensors.