500 Rule Calculator
Find the longest shutter speed for sharp, trail-free stars with any lens.
The 500 Rule for Astrophotography →
What is the 500 rule?
The 500 rule is a quick way to find the longest exposure you can use before the stars blur into short trails from the Earth's rotation. You divide 500 by your full-frame-equivalent focal length, and the result is the maximum shutter time in seconds. A 20 mm lens on full frame gives about 25 seconds.
500 rule vs 300 rule vs NPF
The 500 rule is the classic guideline. The stricter 300 rule suits high-resolution sensors or viewing images at 100%. The NPF rule is more precise still, factoring in aperture and pixel pitch, and usually gives shorter times for truly pinpoint stars. This tool shows both the 500 and 300 figures.
Why sensor size matters for star trails
Because the rule uses the full-frame-equivalent focal length, a crop sensor gives a shorter maximum shutter time - the tighter field of view magnifies star movement. Select your sensor so the limit is correct, and pair a fast wide-angle lens with a high ISO to keep exposures short and stars sharp.
How this calculation works
Formula: t_max = 500 / (f × crop)
where:
f= focal lengthcrop= crop factor
Sources:
FAQ
500 rule or 300 rule?
The 500 rule is the classic guideline; the 300 rule is safer for high-megapixel cameras or when you view images at 100%.
Does sensor size matter?
Yes - the rule uses the full-frame-equivalent focal length, so a crop sensor gives a shorter maximum shutter time.
What is the NPF rule?
The NPF rule is a more precise alternative that also factors in aperture and pixel pitch; it usually gives shorter exposures than the 500 rule for pinpoint stars.
Does a wider lens let me expose longer?
Yes. Shorter focal lengths allow longer shutter times before stars trail, which is why astro shooters favour fast wide-angle lenses.