Star Trail

The longest exposure that still renders stars as points, not streaks. The NPF rule is sharper than the classic 500 rule because it accounts for your aperture and sensor density.

Max shutter — NPF rule — Recommended. Points-of-light at pixel scale.

500 rule (classic, looser): —
Recommended ISO (dark sky, Milky Way): 1600–6400

Focal length mm Aperture ƒ/

Sensor Pixel pitch µm

Pixel pitch = sensor width ÷ horizontal pixels, in microns. A 24MP full-frame (6000px wide) is ≈ 5.94 µm. Denser sensors (more megapixels) need shorter exposures.

500-rule strictness

Rule number — 300 strict · 500 standard · 600 loose