Technique

The 500 Rule vs the NPF Rule: How Long Can You Expose Before Stars Trail?

By Still DarkPublished Sep 11, 2026Updated Sep 11, 202613 min read

Without a tracker, every night-sky photo trades light against sharpness. Leave the shutter open too long and the stars turn from points into dashes. Two rules of thumb tell you where to stop. The 500 rule for astrophotography is a quick estimate from the film era, and it gives exposures that are too long for modern cameras. The NPF rule takes your lens and your sensor's pixel size into account and usually gives about half the time. Here's where both come from, what they give for common gear, and how to test your own camera in five minutes so you have a number you can rely on.

Why stars trail

A camera on a tripod turns with the Earth. Earth turns once relative to the stars every 23 h 56 min 4 s (a sidereal day), so the sky drifts westward at just over 15° per hour, which is 15.04 arcseconds for every second the shutter is open. In a 20-second exposure a star on the celestial equator moves about 300 arcseconds. That's a tenth of a degree, or a sixth of the Moon's diameter.

Not every star moves that fast. The sky rotates about the celestial pole, so a star's apparent speed is the equatorial rate times the cosine of its declination. Orion's Belt, at declination −1°, moves at full speed. Vega, at +39°, moves at 78% of it, and Polaris, at +89°, hardly moves at all. Every rule below assumes the worst case, the equator, unless it includes a declination term.

Whether that motion shows as a trail depends on how much sky each pixel covers. A 24 mm lens on 5.9 µm pixels gives about 51 arcseconds per pixel, and on 4.4 µm pixels it gives 38. Smaller pixels turn the same motion into a longer trail, which is why the older rule stopped working.

The 500 rule for astrophotography

The 500 rule is the one most people learn first. Divide 500 by the focal length of the lens, in full-frame-equivalent millimetres, and the answer is the longest exposure in seconds before stars visibly trail. On a crop-sensor camera, multiply the focal length by the crop factor first.

• 14 mm: 500 ÷ 14 ≈ 36 s.
• 24 mm: 500 ÷ 24 ≈ 21 s.
• 24 mm on APS-C (36 mm equivalent): 500 ÷ 36 ≈ 14 s.

It comes from a "600 rule" used with film, where grain and small enlargements set the limit. Digital shooters found 600 too loose and cut it to 500. The "400 rule" and "300 rule" are tighter versions for denser sensors or stricter standards. None of them takes the camera into account. A 12 MP body and a 61 MP body get the same answer, even though the stars cross five times as many pixels on the second.

Here's what that means in pixels. At the 500-rule time, a star on the equator moves 313 arcseconds through a 24 mm lens. That's about 6 pixels on a 24 MP full frame, 8 on a 45 MP full frame and 9 on a 24 MP APS-C body (computed). At 100% those are short dashes. Downsized for a phone they disappear, which is why people still use the rule.

The NPF rule formula

The NPF rule was published by Frédéric Michaud of the Société Astronomique du Havre (SAH) in 2010. The letters are the three inputs: N for the f-number, P for pixel pitch, F for focal length. The simplified version is:

t (seconds) = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel pitch in micrometres, and f the focal length in millimetres (actual, not equivalent, because the pixel pitch already accounts for the sensor size).

The aperture term is there because a slower lens produces a larger diffraction blur, so a star is already a small disc and can drift a little further before it looks stretched. The pixel term does the same for the sensor. With bigger pixels a star can move further before it shows.

The SAH also gives a complete version with a declination term and an accuracy factor:

t = k × (16.9 × N + 0.10 × f + 13.7 × p) ÷ (f × cos δ), where δ is the lowest declination in the frame and k is a tolerance factor from 1 (strictest) to 3. Calculators usually carry the unrounded constants 16.856, 0.0997 and 13.713. With k = 1 at the equator it returns about half the simplified figure, so the simplified rule sits close to k ≈ 2 (computed). Pointing at declination 45° lets you multiply by 1.41, and at 60° by 2.

Working out your pixel pitch

Divide the sensor width by the number of pixels across it. The result is in millimetres, so multiply by 1,000 for micrometres. You only need to do this once per camera.

• 24 MP full frame (36 mm, 6,000 px): 36 ÷ 6,000 = 6.0 µm. Many 24 MP bodies are 35.9 mm and 6,048 px, which gives 5.9 µm.
• 45 MP full frame (36 mm, 8,256 px): 36 ÷ 8,256 = 4.36 µm.
• 61 MP full frame (35.7 mm, 9,504 px): 3.76 µm.
• 24 MP APS-C (23.5 mm, 6,000 px): 3.9 µm.

500 rule vs NPF rule: the numbers

All times in seconds, at f/2.8, for a target on the celestial equator. NPF uses the simplified formula. Values are computed and rounded to the nearest 0.5 s. The APS-C 500-rule column uses the 1.5× equivalent focal length.

The pattern is the same in every row. NPF comes out at 45–55% of the 500 rule on a 24 MP full frame and nearer 40% on dense sensors. Instead of six to nine pixels of trailing, the simplified NPF allows about three (computed for 24 mm f/2.8, 5.9 µm).

Quick reference: NPF for common lenses

Simplified NPF, seconds, at the equator. Computed and rounded to the nearest 0.5 s.

Opening a 24 mm lens from f/2.8 to f/1.4 shortens the NPF time by two seconds. That's not much to give up for four times the light.

What "acceptable" trailing means

There's no physical threshold where a star becomes a trail. It depends on how the picture will be shown and how closely people will look at it. We think the 500 rule is fine for a phone screen and wrong for a print.

• Phone screen or social feed: downsized to two or three megapixels, six to nine pixels of trailing shrink to one. The 500 rule is fine.
• A 4K display or a 12-inch print: about a third of a 24 MP frame's resolution. Three pixels of trailing are invisible, and eight are borderline.
• A large print or 100% view: every pixel shows. Use NPF, or the complete formula with k = 1 if the stars are the main subject.

The trade-off: less time, same light

Halving the shutter time costs one stop. There are four ways to get it back.

1. Raise the ISO. ISO 3200 to 6400 costs little on current sensors, and stacking makes up for it anyway. The ISO and histogram guide explains why.
2. Open the aperture. f/2.8 to f/1.8 is 1⅓ stops. Wide open, most lenses show coma in the corners. Stopping down a third to a half stop usually fixes most of it.
3. Stack frames. Shoot 10–20 identical NPF-length frames and register them in Sequator, Starry Landscape Stacker or DeepSkyStacker. Noise falls with the square root of the frame count. Ten frames give about 3.2× less noise, roughly the gain of dropping from ISO 6400 to 640, and every frame is sharp. The software aligns sky and ground separately.
4. Use a star tracker. A tracking mount turns the camera about an axis aligned with the celestial pole at the sidereal rate, which cancels the rotation. At wide focal lengths you can shoot exposures of 1–4 minutes at ISO 400–800. The foreground blurs instead, so you shoot it separately with the tracker off and blend the two.

Test your own lens in five minutes

The rules are estimates. Your lens, your sensor and how much trailing you'll accept are specific to you, so test once and use that number.

1. Point at the celestial equator, where trailing is fastest. In northern winter use Orion's Belt (declination −1°). In summer use Altair (+9°) or the Milky Way core in Sagittarius (about −29°, moving at 87% of equatorial speed).
2. Focus carefully on a bright star at 100%. A soft star hides trailing.
3. Shoot 10, 15, 20 and 25 seconds at your usual focal length and aperture, at any ISO that shows a star field.
4. View at 100% in the centre and a corner. Find the longest exposure where a star is still round.
5. Write it down. Use that number for that lens instead of either rule.

What to do with the time you have

A frame-by-frame plan for a typical 24 mm f/2.8 lens on a 24 MP full frame, where NPF allows about 11 s.

What this means for a panorama

A Milky Way arch is usually stitched from a row of 24–50 mm frames rather than one ultra-wide shot, because the longer lens records more detail per degree of sky. NPF makes those frames short. 35 mm f/2.8 allows about 8 s on a 24 MP full frame, and 50 mm only 5.5 s. That's why panorama shooters use a fast 24 or 35 mm at f/1.4–f/2, accept ISO 6400–12800, or stack two or three frames per panel before stitching. The arch moves while you shoot, so work quickly and keep the same exposure for every panel.

Common mistakes

• Using the crop-sensor focal length in the 500 rule without multiplying by the crop factor: 500 ÷ 18 mm gives 28 s on an APS-C body where 18 s is the answer.
• Using the equivalent focal length in NPF. NPF uses the actual focal length, because the pixel pitch already accounts for the sensor.
• Applying the declination bonus to a wide frame. A 14 mm lens aimed at Polaris still has stars at declination 40° in its corners, so use the lowest declination in the frame.
• Judging trailing on a soft image. Fix focus first, then test the shutter.
• Pixel-peeping a photo meant for a phone. Match the rule to where the photo will be seen.

FAQ

Sources and further reading

• Société Astronomique du Havre — La Règle NPF (Frédéric Michaud's original 2010 rule: the simplified formula, and the complete version with declination and the k factor).
• Wikipedia — Sidereal time (sidereal day of 86,164.09 s, which gives 15.04 arcseconds per second).
• Fstoppers — Using the NPF Rule for Photographing Night Skies (why the 600/500/400 rules ignore sensor resolution; worked 30 MP example).
• Light Stalking — What Is The NPF Rule (computing pixel pitch from sensor width and pixel count; 24 mm f/2.8 comparison).
• PetaPixel — How to Avoid Star Trails by Following the 500 Rule (the 500 rule, the 600 and 400 variants, and print size versus megapixels).
• Lonely Speck — Advanced Astrophotography Shutter Time Calculator (pixel-tolerance approach; declination and trailing at 100%).