The NPF Rule is a stricter formula for calculating the maximum shutter speed before star trailing, accounting for aperture and sensor pixel density in addition to focal length.
The NPF Rule exists because the 500 Rule was built for film and early digital sensors and no longer holds up. NPF feeds in aperture and the sensor's pixel pitch alongside focal length, producing a shorter, stricter maximum shutter time that actually keeps stars as points on a high-resolution body viewed at full size. The trade-off is that it is not mental maths: the full formula needs a calculator or an app, and it usually returns something near half the 500 Rule's number.
For a photographer who prints large or crops into the frame, that stricter limit is the difference between stars that survive close inspection and stars that turn out to be tiny commas. For a quick frame that will only ever be seen small on a phone, the looser 500 Rule is often good enough and buys a useful stop of extra light.
At 20mm, f/2.8, on a 45-megapixel full-frame sensor, the 500 Rule allows 25 seconds. The NPF Rule, feeding in that aperture plus the sensor's roughly 4.3-micron pixel pitch, returns closer to 12 seconds for genuinely pinpoint stars. That is a full stop of light difference, so the NPF frame at 12 seconds needs ISO 6400 where the 500 Rule frame used ISO 3200, trading a little more noise for tighter stars.
A photographer shooting a foreground-heavy composition that will only be seen at web size might take the 25-second frame and accept slight trailing. One planning a large print of the same scene shoots at the NPF limit, or takes several 12-second frames and stacks them to recover the lost light without lengthening any single exposure. Focus is set once on a bright star with magnified live view and then taped down, because refocusing between stacked frames would shift the star sizes partway through the sequence.
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