ImageReasonAstrophotography and Night Capture
What shutter time keeps stars point-like, untracked?
Enter your focal length, aperture, pixel pitch, and sky declination for a conservative NPF-derived starting shutter time, compared against the simpler 500 rule.
Answer
Visualize
Calculate to see the NPF/500-rule scale.
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| Setting | NPF time |
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Plan
Calculate a result to generate a copyable plan.
Pixel pitch matters as much as focal length
The 500 rule is quick, but it treats a 24-megapixel camera and a much denser sensor as though they show star movement equally. This planner uses the simplified NPF relationship to begin from the real focal length, aperture, pixel pitch, and the part of the sky being photographed. It returns a starting exposure for an untracked nightscape, then makes the practical uncertainty visible: the correct time is the one that holds enough shadow signal while your own close inspection accepts the remaining star shape.
At 24mm, f/2.8, and 5.9 micrometre pixels, the standard simplified NPF result near the celestial equator is about 11.5 seconds. The 500 rule would suggest about 20.8 seconds on full frame. That difference is not a promise that either result is perfect. Start with a conservative 9.2-second version, then bracket below and above it. A longer time can improve foreground and shadow signal, but magnifies star movement. A faster lens increases the simplified NPF allowance while changing depth and aberrations. Review the actual stars at 100% before committing to a sequence.
This is a planning calculation, not a certification. Confirm the relevant camera, lens, card, printer, contract, location, or workflow constraint in the real setup before relying on the result. It does not determine ISO, account for seeing, or work for tracked or telescope imaging.
FAQ
- Why does the 500 rule give a much longer exposure?
- It relies on effective focal length only. This planner also makes pixel pitch, aperture, and sky position explicit, so it is usually the more useful starting point for digital nightscapes.
- Is this a guarantee?
- No. The result is bounded by the inputs and assumptions shown on the page. It is designed to support a better next decision, not replace a test, measurement, proof, or professional judgement where one is needed.