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Find the longest shutter speed that keeps stars as points with the 500 rule. Enter focal length and sensor crop factor to get seconds before star trails.

📘 How to Use

  1. Enter your lens's actual focal length in millimeters
  2. Select the crop factor that matches your camera sensor
  3. Choose the 500 or 300 rule to read the maximum exposure seconds

Astro 500 Rule Calculator

mm

The lens's actual focal length (8–1200mm)

500 is lenient; 300 keeps stars tighter as points

nights_stay Results

Max exposure time

—

Full-frame equivalent focal length

— mm

※ 500 rule: max seconds = rule number ÷ (focal length × crop factor)

※ The 500/300 rule is an approximation for untracked, fixed-tripod shots; it ignores pixel density and sky direction, so high-resolution sensors or stars near the celestial equator may need shorter exposures

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Astro 500 Rule Calculator｜Longest Shutter Speed Before Star Trails

Shoot the night sky without a tracker and leaving the shutter open too long turns stars into streaks. This 500 rule calculator takes your focal length and sensor crop factor and returns the longest exposure that still records stars as points, plus the full-frame equivalent focal length.

💡 About this tool

The hardest call in nightscape photography is how many seconds you can expose. Earth's rotation drags the stars across the frame, and the longer your focal length, the sooner that drag shows up as trailing. The 500 rule answers it with one division: max seconds = 500 / (focal length × crop factor). It is fast, easy to memorize, and good enough to get you shooting.

A 24mm lens on full-frame gives 500 / 24 ≈ 20.83s. Put the same lens on a 1.5× APS-C body and you get 500 / (24 × 1.5) ≈ 13.89s, because the smaller sensor crops tighter and magnifies star motion. The calculator handles that conversion for you, so you never have to work out the full-frame equivalent by hand.

When you want tighter pinpoint stars, switch to the 300 rule (divide 300 instead of 500). It is the safer choice for high-resolution bodies and for shots you plan to crop hard or print large.

🧐 Frequently Asked Questions

Do I enter the full-frame equivalent or the focal length printed on the lens? Enter the actual focal length marked on the lens. You pick the crop factor in a separate field, so there is no need to convert twice.

Why is Canon APS-C 1.6× instead of 1.5×? Sensor dimensions vary by maker; Canon's APS-C is slightly smaller than the 1.5× sensors from Nikon, Sony, Fujifilm and Pentax. The tool lets you pick 1.5 or 1.6.

I followed the 500 rule but stars still trail slightly. The 500 rule is a lenient estimate and ignores pixel pitch. On high-megapixel sensors, switch to the 300 rule, or recheck with the NPF rule, which factors in aperture, pixel pitch and the part of the sky you are shooting.

Is the limit the same for every direction in the sky? No. Stars near the celestial equator move fastest, while stars near the celestial pole barely move. The 500 rule ignores direction, so give yourself a shorter margin when shooting toward the equator.

📚 Why focal length drives the limit

The trailing limit comes straight from Earth completing one rotation roughly every 24 hours, which moves stars about 15 arcseconds per second across the sky. A longer focal length turns that same angular motion into more pixels of travel, so telephoto framing demands shorter exposures. The 500 rule compresses this geometry into a single divide rather than a full physical model, which is exactly why photographers can run it in their head at the trailhead and why it remains the standard starting point for nightscapes.