How the star exposure calculator works
The rule and its arithmetic
The 500 rule: maximum shutter = 500 ÷ (focal length × crop) — the time before Earth's rotation smears stars across the pixel grid. The stricter 400 rule buys pinpoint stars on high-resolution sensors; the declination factor (polar targets move slower) refines further. All of it is 15°/hour geometry.
The Milky Way recipe
Widest fast lens (14mm f/2.8-class), 15–20 seconds, ISO 3200, manual focus on a bright star (or live-view 10× magnification), white balance ~4000K, and the galactic core in frame (season and direction vary — apps map it). The recipe is boring on purpose: it works, so creativity goes to composition.
Beyond single frames
Star trackers counter-rotate the camera and enable 2–5 minute exposures at low ISO — the step-change in image quality that serious night photography eventually buys. Stacking (multiple frames averaged in software) achieves similar noise reduction free — 8 frames of 20 seconds stack into one clean 160-second-equivalent.