Star Trail Calculator: NPF and 500 Rule
Find the longest shutter speed before stars trail, by the NPF rule with pixel pitch and aperture and by the 500 rule, plus star drift in pixels for your camera.
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About this toolWhat is the NPF and 500 Rule Calculator?
Stars move across the sky as the Earth turns, so an untracked night photo can only stay open so long before each star becomes a short line. This calculator gives that limit two ways: the NPF rule, which accounts for your aperture and the size of your pixels, and the classic 500 rule (with stricter 400 and 300 variants), which uses only focal length and crop factor.
Pick your sensor, enter its megapixels, focal length and aperture, and optionally the declination of what you are shooting. Alongside the times you get the speed at which stars drift across the sensor in micrometres and pixels per second, and the length of the trail each rule produces, so you can see why the 500 rule looks soft on modern high-resolution cameras.
- Simplified NPF rule with pixel pitch from sensor size and megapixels
- 500, 400 and 300 rule variants with crop factor
- Declination correction for targets away from the equator
- Star drift in micrometres and pixels per second
- Trail length in pixels at each recommended time
- Comparison table for common focal lengths
How to use the NPF and 500 Rule Calculator
- Choose your sensor format and enter the camera's resolution in megapixels.
- Enter the actual focal length and the aperture you will shoot at, usually wide open.
- Leave declination at 0° for the worst case, or enter the target's declination (Milky Way core about −29°, Orion about 0°, Polaris +89°).
- Read the NPF time for pin-sharp stars and the 500 (or 400, 300) rule time for comparison.
- Check the trail length in pixels and the table of other focal lengths, then set the nearest shutter speed at or below the NPF figure.
Worked examples
Milky Way with a 20 mm lens
Input
Full frame, 24 MP, 20 mm, f/2.8, declination 0°
Output
NPF 13.9 s; 500 rule 25 s
Pixel pitch is 6.0 µm. At 25 s a star would drift about 36 µm, roughly six pixels, so the 500 rule gives visible trails at 100%.
APS-C wide angle
Input
APS-C 23.5 × 15.6 mm, 24 MP, 16 mm, f/2, declination 0°
Output
NPF 11.7 s; 500 rule 20.4 s
Smaller 3.9 µm pixels shorten the NPF time, and the crop factor of 1.53 shortens the 500 rule. At 20.4 s a star would cross about six pixels.
Galactic core at its declination
Input
Full frame, 24 MP, 24 mm, f/2.8, declination −29°
Output
NPF 13.2 s instead of 11.6 s at the equator
Stars at −29° move at cos 29° = 0.87 of the equatorial rate, gaining about 14% more time.
Background and insight
The sky turns once per sidereal day, 86,164 seconds, an angular rate of 7.29 × 10⁻⁵ radians per second. A star on the celestial equator therefore moves across the sensor at f × 7.29 × 10⁻⁵ millimetres per second: about 1.46 µm/s with a 20 mm lens. Stars at declination δ move slower by cos δ, which is why stars near the pole make tight circles while those near the equator streak most.
The 500 rule, t = 500 ÷ (focal length × crop factor), dates from 35 mm film and modest enlargements. On the celestial equator it lets a star drift about 36 µm on full frame (24 µm on APS-C), six pixels on a 24 MP full-frame sensor and more on a 45 MP one, which shows as short dashes at 100%. The NPF rule, credited to Frédéric Michaud of the Société Astronomique du Havre, sets the limit from the size of a star's image instead: its widely used simplified form is t = (35 × N + 30 × p) ÷ f, with N the f-number, p the pixel pitch in micrometres and f the focal length. A larger pixel or a smaller aperture (bigger star blur) allows longer exposures.
Pixel pitch comes from the sensor area divided by the pixel count: √(36 × 24 mm² ÷ 24 million) = 6.0 µm for a 24 MP full-frame sensor. For targets away from the celestial equator the calculator also divides the NPF time by cos δ, capped at 85° so the answer stays finite near the pole. The drift and trail-length figures are straight geometry and do not depend on either rule.
Common use cases
Milky Way landscapes
Pick the longest exposure that keeps stars round before stacking or raising ISO.
Choosing a lens for night photography
Compare how much time a 14 mm, 20 mm or 24 mm lens allows on your camera.
Star trails on purpose
Know the minimum exposure that clearly shows motion, or how long trails will be.
Planning tracker use
See when the untracked limit is too short for the noise you can accept and a star tracker would help.
Tips and limitations
- Use the real focal length and real sensor size. The crop factor matters for the 500 rule but the NPF rule already uses the pixel pitch.
- Lenses are often softest wide open; stopping down from f/1.8 to f/2.8 slightly lengthens the NPF time and sharpens the corners.
- Stars near the edge of a wide frame on the celestial equator trail most; check corners at 100% before trusting a shorter rule.
- If you resize images for the web, a rule closer to 500 can be acceptable. For large prints use NPF or shorter.
- For longer total exposure, shoot several frames at the NPF time and stack them, or use a tracking mount.
Frequently asked questions
What is the NPF rule formula?
The simplified form is t = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel pitch in micrometres and f the focal length in millimetres. It gives the exposure in seconds for stars that look round at 100%.
Why does the 500 rule give star trails?
It was made for film viewed as small prints. On a modern sensor with 4 to 6 µm pixels, 500 ÷ focal length lets a star move several pixels, which shows as short trails when you zoom in.
How do I find my camera's pixel pitch?
Divide the sensor area by the number of pixels and take the square root: √(width × height ÷ pixels). The calculator does it from the sensor size and megapixels; 24 MP full frame is 6.0 µm.
Does aperture affect how long I can expose the stars?
In the NPF rule, yes. A larger f-number makes each star's image slightly larger, so a little more motion goes unnoticed. In the 500 rule it does not.
Can I expose longer when shooting toward the pole star?
Yes. Stars move at a rate proportional to cos(declination), so near Polaris they barely move. Enter the declination to scale the NPF time; the calculator caps it at 85°.
Is the crop factor used in the NPF rule?
Not directly. Crop factor is in the 500 rule. The NPF rule uses the actual focal length and the pixel pitch, which already reflect the sensor size.
Is there an in-depth guide to the NPF and 500 Rule Calculator?
Yes. “NPF Rule vs 500 Rule: Sharp Stars Without a Tracker” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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