500 Rule Astrophotography Calculator
The Earth turns under the sky all night, so a long exposure smears stars into short streaks. The 500 rule is the simplest field guide to the longest shutter speed that still records stars as points: divide 500 by the full-frame-equivalent focal length. This calculator applies the rule from your lens focal length and sensor crop factor, and also shows the stricter 300 and 400 rule results for high-resolution sensors. Use it to set your maximum exposure for nightscapes and Milky Way shots before reaching for a star tracker.
500 rule formula
Equivalent focal length = focal length * crop factor
500 rule max time = 500 / equivalent focal length (seconds)
Tighter: 400 / equivalent, 300 / equivalent
Crop factor: full-frame 1, APS-C about 1.5, Micro Four Thirds 2
The result is the longest exposure before stars visibly streak. Use a stricter divisor for pixel-level sharpness on high-megapixel sensors.
Night sky exposure context
- The sky rotates about 15 degrees per hour, so longer lenses trail sooner.
- Multiply the lens focal length by the crop factor before applying the rule.
- High-resolution sensors show trailing earlier; the 300 or NPF rule is tighter.
- If the frame is too dark, open up, raise ISO, or stack frames instead of exposing longer.
- A star tracker removes the limit entirely by following the sky's motion.
500 rule: frequently asked questions
What is the 500 rule in astrophotography?
The 500 rule is a quick guide for the longest exposure that keeps stars looking like points rather than streaks. The maximum shutter time in seconds equals 500 divided by the full-frame-equivalent focal length: t = 500 / (focal length times crop factor). A 20 mm lens on a full-frame body gives 25 seconds.
Why divide by the crop factor's equivalent focal length?
Star trailing depends on how much the sky moves across each pixel, which scales with the effective focal length. A crop sensor magnifies the image, so you multiply the lens focal length by the crop factor (1.5 for most APS-C, 2 for Micro Four Thirds) before dividing 500 by it.
Is the 500 rule or the 300 rule better?
The 500 rule is generous and can show slight trailing on high-resolution sensors when viewed at 100 percent. Some photographers use 300 or 400 for tighter stars. For pixel-level sharpness the NPF rule accounts for aperture and pixel size. Pick the divisor that matches how critically you inspect your images.
Does the 500 rule depend on aperture or megapixels?
No. The 500 rule uses only focal length and crop factor, so it ignores aperture and sensor resolution. That makes it fast but approximate. High-megapixel cameras show trailing sooner, which is why the NPF rule, which includes pixel pitch and aperture, is more precise.
What does the result mean in practice?
The result is the longest single exposure before stars visibly streak. Use a shutter speed at or below it. If that exposure is too dark, open the aperture, raise the ISO, or stack multiple frames rather than exposing longer, which would trail the stars.
Official sources
- NASA: Earth rotation and celestial motion references.
- CIPA: Camera and Imaging Products Association standards.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.