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NPF Rule Calculator for Astrophotography

Shooting the night sky on a tripod? Find the longest shutter time that keeps stars as points, using your lens aperture and your sensor's pixel size.

Max exposure (NPF)

13.9 s

Pixel pitch used: 6 µm

500 rule (for comparison)

25 s

How it works

  1. Enter the focal length of your lens and its aperture (f-number).
  2. Give the pixel pitch in micrometres, or enter the sensor width and the image width in pixels to compute it.
  3. Optionally add your crop factor to compare with the 500 rule.
  4. Use the result as the longest exposure, then test a frame and zoom in on the stars.

What people use it for

  • Milky Way on a tripod

    Pick a shutter time that keeps stars sharp without a tracker.

  • High-resolution cameras

    See why a 45-megapixel body needs shorter exposures than the 500 rule suggests.

  • Comparing lenses

    Check how a wider aperture or a longer focal length changes the limit.

Questions

What is the NPF rule?
Maximum exposure in seconds = (35 × f-number + 30 × pixel pitch in µm) ÷ focal length in mm. It was developed by Frédéric Michaud of the Société Astronomique du Havre and takes the pixel size into account, which the 500 rule ignores.
How do I find the pixel pitch?
Divide the sensor width in millimetres by the image width in pixels and multiply by 1,000. A 36 mm sensor with 6,000 pixels across has a 6 µm pitch.
Is the result exact?
It is a guideline for stars near the celestial equator, where they move fastest. Stars near the poles trail more slowly, and some people accept slightly longer trails, so check a test frame at 100% zoom.