Stars without trails (NPF)
The earth turns, so stars move across your sensor. Enter your camera, lens and aperture: the NPF rule gives the longest time before stars become short lines.
How it works
NPF rule: time = (35 × f-number + 30 × pixel pitch in µm) ÷ focal length, in seconds. Stars near the celestial pole move slower: the time is divided by the cosine of their declination.
The 500 rule (500 ÷ full-frame-equivalent focal length) was made for film and low-resolution cameras; on today's sensors it gives visible trails.
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