The NPF Rule is the most accurate method for calculating the maximum shutter speed in astrophotography before stars begin to trail. Unlike the older "500 Rule" or "600 Rule," the NPF formula takes into account your camera's sensor resolution (pixel pitch) and lens aperture, making it essential for modern high-resolution cameras.
The NPF Formula Explained
Where Aperture is your f-number (e.g., 2.8), Pixel Pitch is in micrometers (μm), and Focal Length is in millimeters (mm). The result is your maximum shutter speed in seconds.
Why NPF is More Accurate Than the 500 Rule
The 500 Rule (shutter speed = 500 ÷ focal length) was developed for film cameras and early digital sensors. Modern cameras with 40-60+ megapixels require shorter exposures to avoid star trailing.
- Accounts for sensor resolution: A 61MP Sony A7R V needs much shorter exposures than a 12MP Sony A7S III at the same focal length.
- Considers aperture: Wider apertures (f/1.4-f/2.8) create larger star images on the sensor, making slight movement less visible.
- More precise results: The 500 Rule can overestimate safe shutter speeds by 50-100% for modern cameras, resulting in oblong stars.
Understanding Pixel Pitch for Your Camera
Pixel pitch is the distance between the centers of adjacent pixels on your camera's sensor, measured in micrometers (μm). Here's how pixel pitch relates to megapixels:
- Large pixel pitch (6-8μm): Lower resolution sensors (12-24MP full frame) - longer exposures possible
- Medium pixel pitch (4-6μm): Standard resolution sensors (24-33MP full frame) - moderate exposure times
- Small pixel pitch (3-4μm): High resolution sensors (45-61MP full frame, APS-C) - shorter exposures needed
Best Settings for Milky Way Photography
- Focal length: 14-24mm for wide Milky Way shots
- Aperture: f/1.4-f/2.8 for maximum light gathering
- ISO: 1600-6400 depending on your camera's high ISO performance
- Shutter speed: Use this NPF calculator for your exact settings
- Focus: Manual focus at infinity, verify with live view magnification
Tips for Getting Sharp Stars Every Time
- Use a sturdy tripod - carbon fiber dampens vibrations better than aluminum
- Turn off image stabilization - it can cause blur on a tripod
- Use a remote shutter release or your camera's 2-second timer
- Focus manually using live view at 10x magnification on a bright star
- Use the "Sharp" accuracy setting for critical work and large prints
- Consider image stacking - combine 10-20 shorter exposures for better noise and sharpness
Celestial Declination and Star Movement
Not all stars move at the same apparent speed. Stars near the celestial equator (declination 0°, including most of the Milky Way core) move fastest across the sky. Stars near the celestial poles (Polaris in the north, Sigma Octantis in the south) appear nearly stationary, allowing longer exposures.
Pro tip: When shooting circumpolar star trails, you can ignore the NPF rule entirely and use exposures of several minutes to capture beautiful star trail circles.