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NPF Rule Calculator: Shutter Speed for Sharp Stars

Find the longest exposure before stars start to trail with the NPF rule, from your lens's focal length and aperture and your sensor's pixel pitch, and compare it with the older 500 rule.

Last updated: May 2026

Maximum exposure (NPF rule)

13.8 seconds

Formula below · 3 sources (petapixel.com, photopills.com, nationalparksatnight.com)

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About this calculator

Stars move across the sky as the Earth turns, so a long exposure turns them into short trails. The NPF rule, developed by Frédéric Michaud of the Société Astronomique du Havre, estimates the longest exposure before trailing becomes visible at the pixel level. Its simplified form:

t = (35 × N + 30 × p) ÷ f

where N is the f-number, p the pixel pitch in micrometres and f the actual focal length in millimetres. It accounts for the sensor's resolution (small pixels show trails sooner) and the lens's aperture (blur from a wide aperture hides a little movement).

Pixel pitch = sensor width in mm ÷ horizontal pixels × 1000. A 24 MP full-frame camera (6000 px across 35.9 mm) is about 6.0 µm; a 45 MP full-frame about 4.4 µm; a 24 MP APS-C about 3.9 µm; a 20 MP Micro Four Thirds about 3.3 µm.

Stars near the celestial pole move more slowly across the sensor, so the time is divided by cos(declination): 0° (the celestial equator, including Orion's belt) is the strictest case; the Milky Way core sits around −29°.

The older 500 rule (500 ÷ equivalent focal length) was set for film and small prints. On modern high-resolution sensors it allows roughly twice as long as NPF and shows visible trailing at 100%; it remains a fine quick estimate for web-size images.

How to use

Enter the focal length on the lens, the aperture you will shoot at, your camera's pixel pitch (see the explanation for typical values) and the sensor format for the 500-rule comparison. Example (the defaults): a 20 mm f/2.8 lens on a 24 MP full-frame camera (5.9 µm): t = (35 × 2.8 + 30 × 5.9) ÷ 20 = (98 + 177) ÷ 20 = 13.75 s, against 500 ÷ 20 = 25 s by the 500 rule. Example 2: 14 mm f/1.8 on a 45 MP full frame (4.4 µm): (63 + 132) ÷ 14 = 13.9 s. Example 3: aiming at the Milky Way core (declination about −29°) with the default lens: 13.75 ÷ cos 29° = 15.7 s.

Frequently asked questions

NPF rule or 500 rule: which should I use?

Use NPF if you print large or view at 100%, especially on cameras above about 20 megapixels; it gives pin-point stars. The 500 rule gives about twice the exposure and more light, with slightly elongated stars that are invisible at social-media sizes. Some photographers split the difference, for example a 300 rule.

How do I find my camera's pixel pitch?

Divide the sensor width in millimetres by the number of pixels across, then multiply by 1000. A Sony A7 III: 35.6 mm ÷ 6000 px × 1000 = 5.9 µm. A Canon R5: 36 mm ÷ 8192 px × 1000 = 4.4 µm. Spec sheets and sites such as DPReview list sensor size and resolution.

What ISO and aperture should I use for the Milky Way?

Shoot as wide open as the lens is sharp (f/1.4–f/2.8), at the NPF exposure, and raise ISO until the histogram is a third to half of the way from the left; ISO 3200–6400 is typical on full frame under dark skies. Shoot RAW, focus manually on a bright star with magnified live view, and use a sturdy tripod with a 2-second or remote release.

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