NPF Star Sharpness Rule Calculator
The NPF rule is the precise successor to the 500 rule for keeping stars as pinpoints. Where the old rule used only focal length, the NPF rule also folds in the lens aperture (N) and the sensor's pixel pitch (P), so it correctly predicts that high-resolution cameras show star trailing sooner. This calculator returns the longest exposure in seconds before stars streak at the pixel level, from your aperture f-number, pixel pitch in microns, and focal length in millimetres. Use it when you inspect nightscapes at full resolution.
NPF rule formula
t = (35 * N + 30 * P) / F (seconds)
N = aperture f-number
P = pixel pitch in microns
F = focal length in millimetres
This common form of the NPF rule targets pinpoint stars at full pixel resolution. It is stricter than the 500 rule because it includes pixel pitch and aperture.
Pinpoint star context
- Pixel pitch equals sensor width in mm times 1,000 divided by horizontal pixels.
- Higher-megapixel sensors have smaller pixels and show trailing sooner.
- Longer focal lengths shorten the allowed exposure quickly.
- The NPF rule is tighter than the 500 rule, so use it for critical pixel-level work.
- If exposure is too short for brightness, open up, raise ISO, or use a star tracker.
NPF rule: frequently asked questions
What is the NPF rule for astrophotography?
The NPF rule estimates the longest exposure that keeps stars as pinpoints by accounting for aperture (N), pixel pitch (P), and focal length (F). The common form is t = (35 times aperture + 30 times pixel pitch) divided by focal length, with pixel pitch in microns and focal length in millimetres. It is stricter and more accurate than the 500 rule.
How do I find my camera's pixel pitch?
Pixel pitch in microns equals the sensor width in millimetres times 1,000, divided by the number of horizontal pixels. For a 36 mm wide sensor with 6,000 pixels across, pitch is 36,000 divided by 6,000, which is 6 microns. This calculator takes the pixel pitch directly as an input.
Why is the NPF rule better than the 500 rule?
The 500 rule uses only focal length, so it ignores both lens aperture and sensor resolution. The NPF rule adds pixel pitch, which captures how a high-megapixel sensor reveals trailing sooner, and aperture, which affects the size of the star's image. The result is tighter, pixel-accurate exposures.
Does a wider aperture allow a longer exposure under the NPF rule?
Yes, slightly. A larger aperture (smaller f-number) makes each star image a touch larger through diffraction terms in the rule, which allows marginally longer exposure before the streak is noticeable. The dominant factor is still focal length and pixel pitch.
Is this exposure for pinpoint or slightly trailed stars?
This form of the NPF rule targets near-pinpoint stars at the pixel level. Some photographers accept a small amount of trailing and use a more relaxed coefficient. If you inspect at full resolution, use this strict result; for web-size output you can expose a little longer.
Official sources
- NASA: Earth rotation and celestial motion references.
- CIPA: Camera and Imaging Products Association standards.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.