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Astro Shutter Calculator

The longest shutter you can use before stars trail into streaks — the classic 500 rule and the sharper, sensor-aware NPF rule. The same engine the Viewpoint app uses, in your browser.

NPF rule (recommended)—
500 rule—
In the app

Catch the Milky Way at the right hour

Sharp stars are step one — the shot is the Milky Way core rising over your foreground. Viewpoint finds the dark window, tells you when and where the core is up, lines it up behind your subject, and works offline in the field. The shutter math here is the same the app runs.

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500 rule vs NPF rule

The 500 rule (500 ÷ effective focal length) is a quick field estimate of the longest exposure before stars visibly trail. The NPF rule is more accurate: it also accounts for your aperture and the sensor's pixel pitch, so it usually gives a shorter, safer time — especially on high-resolution bodies. Use NPF for pinpoint stars; the 500 rule when you just need a ballpark.

Finding your pixel pitch

Pixel pitch (in microns) ≈ sensor width in mm ÷ horizontal pixel count × 1000. A full-frame 24 MP camera is about 5.9 µm; a 45 MP full-frame is about 4.4 µm; many APS-C bodies are 3.8–4.0 µm.

Related

For trail length on purpose, see the Star trail calculator. Both are in the Viewpoint app.

For the physics behind the 500 and NPF rules, read The 500 rule vs the NPF rule for pinpoint stars.

More calculators

Depth of fieldNear & far focus limits and total depth of field. Hyperfocal distanceThe focus distance that's sharp to infinity. Exposure value (EV)A single value from aperture, shutter and ISO. ND & long exposureYour shutter time with a neutral-density filter. Star trailsTrail arc length and total sky rotation.