Photography Calculators

Astrophotography

500 Rule Calculator

The 500 rule is a field fudge: t ≈ 500 / (crop × focal length) seconds before stars become dashes on a small print. High-megapixel bodies want the 300 or 200 rule. It ignores declination and pixel size — NPF is tighter. Always take a test shot at 100% before you lock the intervalometer.

Inputs

Units
mm

Barrel focal length. A 24 mm on full frame is the usual Milky Way starting lens.

×

1 full frame, 1.5 APS-C, 2 Micro Four Thirds.

Results

500 rule shutter
20.83s
300 rule shutter
12.50s
200 rule shutter
8.33s

Recommended gear

Calculation method

Divide 500 by crop factor times focal length in millimetres to get a starting untracked shutter in seconds. The 300 and 200 versions use 300 and 200 in the numerator for tighter trails on high-MP sensors or large prints. A 24 mm on full frame is about 21 s (500), 12.5 s (300), 8.3 s (200). Crop bodies divide by a larger number and get less time.

Exact formula

t500 = 500/(crop×f). t300 = 300/(crop×f). t200 = 200/(crop×f). Limits: Declination, pixel pitch, tracking and optical quality omitted; NPF is the sharper next step.

ISO after the time is set

Fix shutter with this rule, shoot wide open, then raise ISO until the histogram is off the left wall. The Milky Way calculator sketches that ISO for you.

Tracked vs untracked

A tracker makes these rules obsolete for the sky. You then expose for skyfog and guiding error, not for star dashes.

Frequently asked questions

Which rule should I use?

Start with 300 on a modern 24 MP+ body. Use 500 only if you view small. Use 200 if you print large or pixel-peep. Then look at NPF if you know pixel pitch.

Stars near the pole vs the equator?

Motion is slower near the pole. The 500 rule assumes a worst-ish field. You can go longer on Polaris; do not on the celestial equator.

Does IBIS help?

Not for earth rotation. IBIS fights your shake, not sidereal motion. Turn it off on a locked tripod so it does not drift.

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