NPF Rule Calculator
Accurate star photography shutter speed calculation
Camera & Lens Settings
15.2s
NPF Rule (Accurate)
20.8s
500 Rule
12.5s
300 Rule
NPF = (35 × N + 30 × P) / f × D
Understanding the NPF Rule
The NPF rule accounts for pixel size and aperture to calculate the maximum exposure time before stars trail. It’s more accurate than the 500 or 300 rules, especially for high-resolution sensors.
N = Aperture (f-number)
P = Pixel pitch in micrometers
f = Focal length in mm
D = Declination factor (1 at equator, lower at poles)
Tips for Sharp Stars
• Use a sturdy tripod and remote shutter release
• Focus manually at infinity using live view zoom
• Shoot RAW for better noise reduction in post
• Use the highest ISO your camera handles well
• Consider image stacking for cleaner results