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Find the longest Milky Way exposure before stars trail

Compare the 500 rule and the NPF rule for star trails, using your sensor, focal length, aperture and optional declination.

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500 rule

NPF rule

How to compare the 500 rule and NPF for a Milky Way shot

  1. Choose your sensor from the list, or choose a custom sensor and enter its width and megapixels.
  2. Enter the focal length of the lens and the aperture you plan to use.
  3. Optional: enter the declination of the part of the sky you photograph, between -89 and 89 degrees.
  4. Compare the two limits. The shorter time keeps the stars smaller on the sensor.

The 500 rule and the NPF rule

The 500 rule divides 500 by the focal length multiplied by the crop factor. A 24 mm lens on a full-frame camera gives 500 ÷ 24 = 20.8 seconds. On an APS-C camera with a 1.5 crop factor, the same lens gives 500 ÷ 36 = 13.9 seconds. The NPF rule is (35 × aperture + 30 × pixel pitch in micrometres) ÷ focal length. The 35 term is the lens part of the blur and the 30 term is the sensor part.

Why NPF is shorter on high-megapixel sensors

More megapixels on the same sensor width means smaller pixels, and a smaller pixel pitch lowers the 30 × pixel pitch term. Take a 24 mm lens at f/2.8. On a 24 MP full-frame sensor with a 6.0 µm pixel pitch, NPF gives (98 + 180) ÷ 24 = 11.6 seconds. On a 45 MP sensor, the pitch is 4.4 µm and NPF gives 9.6 seconds. On a 61 MP sensor, the pitch is 3.8 µm and NPF gives 8.8 seconds. The 500 rule stays at 20.8 seconds for all three, because it does not use the pixel count.

Declination

Stars near the celestial pole move more slowly across the frame, so their trails are shorter for the same exposure. The calculator divides each limit by the cosine of the declination. At 60 degrees the cosine is 0.5, so both limits double. A 24 mm lens on a full-frame sensor then gives 41.7 seconds with the 500 rule. Leave the field empty to use the celestial equator.

Tips for better results

  • Each full stop of aperture adds 35 ÷ focal length to the NPF limit, which is 1.5 seconds for a 24 mm lens.
  • Set the focal length you shot at, because a zoom lens changes it as you turn the ring.
  • Check a test frame at the limit and inspect the corners at 100 % magnification.
  • Use the declination of the centre of the frame, not of the horizon.

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Frequently asked questions

What is the 500 rule for?

It gives a quick estimate of the longest exposure before stars begin to trail on a given lens and sensor. It is a rule of thumb rather than a measurement, and it does not use the megapixel count or the aperture.

Why do the two results differ?

The 500 rule uses only the focal length and the crop factor. NPF also uses the aperture and the pixel pitch, so on high-megapixel cameras it gives a shorter time than the 500 rule.

How is pixel pitch estimated?

The calculator divides the sensor width by the number of pixels across it. The pixel count across the width is the square root of the megapixels times the aspect ratio. Presets use their own aspect ratio, and a custom sensor assumes 3:2.

What does declination change?

Declination is how far the part of the sky you photograph lies north or south of the celestial equator. Near the pole, stars move more slowly across the frame, so both limits become longer.

Is the NPF rule exact?

No. It is a published estimate for keeping stars point-like on most lenses. Lens quality, focus and the corners of the frame can change the result, so check a test frame.

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