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The NPF rule is somewhat more "advanced" and more accurate than the 600 or 500 rule for astrophotography - both the latter tend to still produce star trails as camera resolution increased in recent years.

The NPF rule is

Shutter speed = (35* Aperture + 30* pixel pitch) / focal lenght

Shutter speed is in seconds, Aperture in f-Stops (you can just insert the value, e.g. 2.8 for f/2.8), pixel pitch (the distance between two pixels on the camera sensor) in ┬Ám and focal lenght in mm.

In the 500 or 600 rule, you have to multiply the focal lenghts by the crop factor of your camera (e.g. 1.5 for APS-C). For the NPF rule, I found no information whether this is necessary.

I assume it is not necessary because for a full frame camera with 24mp, the pixel pitch is larger than for a 24mp APS-C camera, which could possibly account for the crop factor.

So: Do I have to multiply the focal lenght by my camera's crop factor in the NPF rule?

  • The 1/500 or 1/600 rule was never meant to be applicable to pixel level scrutiny. It was meant to be applicable for a display size of 8x10 inches viewed from a distance of 10-12 inches by a person with 20/20 vision. – Michael C Aug 5 at 19:57
  • @MichaelC Interesting, I did not know this. I however found that the rule gives acceptable results if you treat is as a 300 rule. – Jonas Aug 5 at 19:59
  • When standard viewing conditions (as outlined above) are considered, the minimum size circle on a FF sensor (before enlargement) a person with 20/20 vision can discriminate from a point is about 30 microns wide. That's about 4-7 pixels wide for a modern sensor with 7-4µm pixel pitch (about 18-50 MP). – Michael C Aug 5 at 20:11
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No, you do not have to account for crop factor with the NPF rule.

Accounting for crop factors with the other rules is a method of accounting for movement across sensor area due to apparent increased magnification (image circle crop). Instead the NPF rule accounts for movement across individual pixels based on their size; which is exactly why it is considered to be more accurate/advanced.

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When you are pixel peeping, the only things that matter are the size of the camera's pixel pitch, the size of your monitor's pixel pitch, and if your eyes are good enough to discriminate a single pixel on your monitor at the distance from which you are viewing the monitor.

All "rules of thumb" based on standard viewing conditions, e.g. an 8x10 inch print viewed by a person with 20/20 vision from a distance of 12 inches, go out the window when enlarging the image, or a portion thereof, to such large ratios. When one is looking at, say, a 24 MP image on a 24" monitor with a pixel pitch of around 96 ppi one is looking at a portion of an approximately 60 x 40 inch display size!

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