How it works
Earth turns, so stars drift across the sensor during a long exposure. Past a certain time the points of light stretch into short lines. The calculator estimates that limit for a camera on a fixed tripod, with no tracking mount.
The NPF rule is the more precise one. It uses the lens focal length, the aperture (f-number) and the size of each pixel, worked out from the sensor size and megapixel count. High-resolution sensors with small pixels show trailing sooner, which is why their limit is shorter.
The 500 rule is the older rule of thumb: 500 divided by the full-frame-equivalent focal length. It was made for film and lower-resolution cameras, so on modern sensors it often allows faint trails. The 300 rule is a stricter version that many photographers use instead.
Stars near the celestial equator, such as Orion, move fastest across the frame; stars near the celestial pole barely move. Choosing where you aim adjusts the result. Treat every figure as a starting point: take a test shot and zoom in on the stars.
Field of view shows how much sky fits in the frame. The full Moon is about half a degree wide, the Big Dipper is about 25 degrees long, and the bright band of the Milky Way across a summer sky needs a wide-angle lens.
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Light pollution and the Bortle scaleWhy city skies hide the stars, and how the 9-step Bortle scale rates darkness.