Astrophotography
Sharp-Star Exposure
The longest exposure you can take before the stars begin to trail — the NPF rule, tuned to your camera and lens.
‹ All tools
Camera & Sensor
Camera body
Lens
Focal length mm
Aperture f-number
Sharpness
Target declination how far from the celestial equator
0° — equator (fastest trailing)0°
Maximum shutter
12.5sec
NPF rule
12.5s
Old 500 rule
20.8s
Pixel pitch
5.9µm
The NPF rule (Frédéric Michaud, Société Astronomique du Havre) weighs aperture, pixel pitch, and focal length — far more reliable than the old 500 rule on today's high-resolution sensors. A sharp starting point; refine in the field.