🌌 Star Trail (500 Rule) Calculator
Pick your sensor and focal length to get the longest shutter time that keeps stars as sharp points, not streaks.
Max exposure before star trails
20.8 sec
24mm full-frame equivalent · crop factor 1×
The "500 rule": longest exposure (seconds) ≈ 500 ÷ (focal length × crop factor), which keeps stars as points rather than streaks as the Earth turns. A wider (shorter) lens allows longer exposures. Modern high-resolution sensors show trailing sooner, so many astrophotographers use 300 or 200 instead. For the sharpest results the NPF rule accounts for aperture and pixel pitch too. 🔒 In your browser.
How the star trail (500 rule) calculator works
As the Earth rotates, stars drift across the frame, so a long exposure records them as short streaks instead of points. The "500 rule" estimates the longest safe shutter: seconds ≈ 500 ÷ (focal length × crop factor). A wider lens sees a larger patch of sky, so each star moves a smaller fraction of the frame and you can expose longer. The crop factor converts your lens to its full-frame-equivalent field of view, so the same 24 mm lens allows a shorter exposure on a crop-sensor body than on full frame.
The 500 rule is lenient and dates from lower-resolution days; on today's high-megapixel sensors stars trail sooner, so many photographers use 300 or even 200. For critical sharpness the NPF rule factors in aperture and pixel pitch. Use a sturdy tripod and a wide aperture, and raise ISO rather than exposing longer if you need more light.
Frequently asked questions
What is the 500 rule in photography?
A guideline for the longest exposure that keeps stars as points: shutter time (seconds) = 500 ÷ (focal length × crop factor). For a 20 mm lens on full frame that's about 25 seconds; longer and stars begin to streak.
How do I calculate max shutter for stars?
Divide 500 by your effective focal length (focal length × sensor crop factor). A 24 mm lens on an APS-C body (1.5×) gives 500 ÷ 36 ≈ 14 seconds. Use 300 or 200 instead of 500 for stricter, sharper results.
Should I use the 500, 300 or 200 rule?
Use 500 for web-sized images or forgiving setups, 300 for a balance, and 200 for high-resolution sensors or when you'll pixel-peep or print large. Lower numbers give shorter exposures and rounder stars.
Does crop factor affect the 500 rule?
Yes — a crop sensor magnifies the view, so stars cross the frame faster relative to it. Multiply focal length by the crop factor (1.5× APS-C, 2× Micro Four Thirds) before dividing into 500. This tool does it automatically from the sensor you pick.
What is the NPF rule?
A more precise formula than the 500 rule that accounts for aperture, pixel pitch and declination for pin-sharp stars. It gives shorter times than the 500 rule; the 500/300/200 rules are simpler field approximations that work well in practice.