500 Rule Calculator
Calculate the maximum exposure time before Earth rotation causes pinpoint stars to blur into elongated star trails.
Recommended Tools & Equipment
Tested hardware and components for high reliability
How the 500 Rule Works
Because the Earth rotates 360 degrees every 24 hours (approx. 15 arcseconds per second), stars move across a digital camera sensor. The 500 Rule estimates the maximum exposure time before star trailing becomes noticeable:
The 500 Rule Equation
Shutter Speed (sec) = 500 / (Focal Length × Crop Factor)
Example: A 24mm lens on a Full Frame camera = 500 / 24 ≈ 20.8 seconds.
Why Modern High-Megapixel Sensors Need the NPF Rule
The classic 500 rule was invented in the 35mm film era. On modern 45MP+ sensors (Sony A7R, Nikon Z8/Z9), individual pixels are smaller than 4 microns, causing stars to cross multiple pixel boundaries in fewer seconds. The NPF Rule factors in pixel pitch and aperture for pixel-perfect sharpness.
Frequently Asked Questions
Why do stars trail faster near the celestial equator?
Stars near the celestial equator (Declination 0°) travel through the largest angular circumference across the night sky, moving significantly faster across your sensor than stars near Polaris (North Star).
What ISO should I use for Milky Way photography?
Most modern full-frame sensors perform best between ISO 3200 and ISO 6400 with the widest aperture available (e.g. f/1.4, f/1.8, or f/2.8).
What is star trailing?
When shutter speeds exceed the motion threshold, point sources of light (stars) streak into short lines rather than appearing as crisp, pinpoint dots.