NPF Rule Calculator
Find the longest shutter speed before stars start to trail on a fixed tripod. Enter your sensor, lens and aperture, or read them from a photo, and compare the NPF rule with the 500 rule.
- Free · no account
- Photos are not uploaded
- Runs in your browser
Longest exposure, NPF rule
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| Rule | Longest exposure | Camera setting | Star drift |
|---|
- Pixel pitch
- –
- Crop factor
- –
- Full-frame equivalent
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Read settings from a photo
Choose a JPEG or RAW photo to fill in focal length, aperture and pixel pitch from its EXIF data. For night shots, the tool also shows how far stars moved during the actual exposure.
JPEG, TIFF and TIFF-based RAW such as DNG, NEF, ARW, CR2, ORF and RW2
Only the metadata is used · nothing is uploaded
No uploads. Calculations and EXIF reading run in your browser.
How to use the NPF rule calculator
Describe your camera
Pick the sensor format and enter the megapixels or image width. The calculator works out the pixel pitch, the main reason high-resolution cameras show trails sooner.
Set the lens and the sky
Enter the real focal length and the aperture you will shoot at. Choose a declination or keep 0° for the worst case.
Use the camera setting
Set the shutter to the suggested standard speed or shorter, then raise ISO or open the aperture to get enough light. Check a corner star at 100% after the first frame.
The NPF rule formula
The NPF rule was developed by Frédéric Michaud of the Société Astronomique du Havre. It estimates the longest exposure before a star visibly moves across the sensor. Unlike the 500 rule, it accounts for the aperture N, the pixel pitch p and the focal length f, which give the rule its name.
Full NPF rule
t = k × (16.856 N + 0.0997 f + 13.713 p) ÷ (f × cos δ)
t is the exposure in seconds, N the f-number, f the focal length in mm, p the pixel pitch in µm and δ the declination of the stars in the frame. The factor k sets how much elongation you accept: 1 for pinpoint stars, up to 3 for slightly elongated stars. This calculator uses the full rule for its main result.
Simplified NPF rule
t = (35 N + 30 p) ÷ f
The simplified version assumes k = 2 and stars on the celestial equator. It is easy to compute in the field and is shown in the comparison table. Because of its larger k, it allows longer exposures than the full rule with k = 1.
Pixel pitch is the sensor width divided by the number of pixels across it. A 24-megapixel full-frame sensor is 6000 pixels wide, so its pitch is 36 mm ÷ 6000 = 6 µm. Star drift in pixels is calculated from the sidereal rotation of the sky: a star at declination δ moves f × t × cos δ ÷ 13 713 on the sensor.
NPF rule vs 500 rule
The 500 rule divides 500 by the full-frame equivalent focal length. It ignores pixel size, so it gives the same answer for a 12 MP and a 61 MP camera. On current sensors, its result often shows short trails when you view the image at 100%. The table compares both rules at f/2.8, declination 0° and k = 1, as calculated by this tool.
| Focal length | 500 rule, full frame | NPF, 24 MP full frame (6.00 µm) | NPF, 45 MP full frame (4.39 µm) | NPF, 26 MP APS-C (3.77 µm) |
|---|---|---|---|---|
| 14 mm | 35.7 s | 9.3 s | 7.8 s | 7.2 s |
| 20 mm | 25.0 s | 6.6 s | 5.5 s | 5.0 s |
| 24 mm | 20.8 s | 5.5 s | 4.6 s | 4.2 s |
| 35 mm | 14.3 s | 3.8 s | 3.2 s | 2.9 s |
| 50 mm | 10.0 s | 2.7 s | 2.2 s | 2.1 s |
Many photographers accept a little more drift than k = 1 for web-sized images. Try k = 2 to see the difference: it doubles the NPF result. The calculator shows star drift in pixels for every rule so you can judge the trade-off yourself.
Reading lens settings from a photo
Choose a photo and the tool reads its EXIF focal length, f-number, exposure time and image size. Pixel pitch comes from the FocalPlaneXResolution tag when the camera writes it, or is estimated from the 35 mm equivalent focal length, which is common for phones. Check the result against your camera's specifications: some cameras write these tags differently, and reduced-size JPEGs describe the saved image rather than the full sensor.
For night photos, the tool also estimates how many pixels a star moved during the recorded exposure at the declination you set. Only the beginning of the file is read (the first 1 MB, or up to 64 MB when the metadata is further in), and nothing leaves your device. HEIC, CR3 and RAF files are not supported. To see every metadata field in a photo, use the EXIF Privacy Checker; for camera usage, the Shutter Count Checker reads supported RAW and JPG files locally.
NPF Rule Calculator FAQ
Is the NPF rule more accurate than the 500 rule?
For digital cameras, usually yes. It includes pixel pitch and aperture, which the 500 rule ignores. It is still an approximation: lens aberrations, atmospheric seeing and how much you enlarge the image all affect what you see.
Which declination should I use?
Use the declination of the part of the sky closest to the celestial equator in your frame. With a wide-angle lens that usually means 0°. For the Milky Way core, about -29° gives a slightly longer exposure. Values above 89° are treated as 89°.
Which k value should I choose?
k = 1 keeps stars as points at 100% view. k = 2 allows drift of about one star diameter, which is often invisible in web-sized images. k = 3 gives slightly elongated stars and suits small prints or quick previews.
Why do my stars still look trailed?
Check for wind, a soft tripod, lens coma in the corners, focus and heavy cropping. A crop enlarges the image and makes the same drift easier to see. Shorten the exposure or reduce k if the trails come from sky rotation.
Does it work for phones and star trackers?
Phone results depend on an estimated sensor size from EXIF and may be less precise. Night modes that stack frames do not follow this rule. The rule is for a fixed tripod; a tracking mount compensates for sky rotation and allows much longer exposures.
Are my photos uploaded?
No. The photo is read locally in your browser, only its metadata is parsed and the file is not modified.