500 Rule Calculator
Calculate the maximum exposure time before Earth rotation causes pinpoint stars to blur into elongated star trails.
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.