Night Sky Exposure Calculator
Calculate ideal exposure settings for astrophotography and star photography.
Use the 500 rule and NPF rule to find maximum shutter speed without star trails.
Includes Milky Way and deep sky settings.
Star Trails vs. Sharp Stars Earth’s rotation makes stars appear to move across the sky. Too long an exposure → stars appear as streaks (trails) instead of points. The goal: expose long enough to collect light, but short enough to freeze stars.
The 500 Rule (Traditional) Maximum shutter speed (seconds) = 500 / (crop factor × focal length in mm) Simple but approximate — works best for full-frame cameras. Full-frame (crop=1.0): 500 / focal_length APS-C Canon (crop=1.6): 500 / (1.6 × focal_length) APS-C Nikon (crop=1.5): 500 / (1.5 × focal_length) Micro Four Thirds (crop=2.0): 500 / (2.0 × focal_length)
The NPF Rule (More Accurate) t = (35 × N + 30 × p) / f Where: N = aperture f-number p = pixel pitch in μm = sensor width (μm) / horizontal pixel count f = focal length (mm) More accurate than 500 rule, especially for modern high-resolution sensors.
Recommended Settings for Milky Way Aperture: as wide as possible (f/1.4–f/2.8) ISO: 1600–6400 (depends on camera) Shutter: use NPF or 500 rule result Focus: infinity (manual focus, live view, focus on bright star) White balance: 3800–4500 K (slightly warm to bring out the colors)
Exposure Triangle for Astrophotography Opening aperture 1 stop doubles light: f/2.8 → f/2.0 (2× light) Doubling ISO doubles brightness (but also noise) Doubling shutter speed doubles light (but risks trailing) For faint objects: stack multiple exposures instead of single long exposure.