Estimate fixed-tripod exposure and star drift using the 500 rule, the NPF rule, and a pixel-level star trail check for your camera and lens.
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500 Rule Astrophotography Exposure Formula
t = R / (CF * FL)
- t is a heuristic exposure estimate in seconds; visible trailing depends on the setup and output size
- R is the rule number (500 standard, 400 or 300 for stricter results, 200 for very high resolution sensors)
- CF is the crop factor of your camera (1.0 full frame, 1.5 APS-C Nikon/Sony/Fuji, 1.6 APS-C Canon, 2.0 Micro Four Thirds)
- FL is the real focal length of your lens in millimeters
The NPF-based mode uses the simplified NPF expression with a declination adjustment. It accounts for aperture and pixel pitch, but is not a sharpness guarantee or the full NPF equation. The NPF rule was developed by Frédéric Michaud of the Société Astronomique du Havre:
t = K * (35*N + 30*p) / (FL * cos(D))
- N is the aperture f-number (for example 2.8)
- p is the pixel pitch in microns, equal to 1000 times the sensor width in mm divided by the horizontal pixel count
- D is the declination of the target in degrees (0 at the celestial equator, where stars move fastest across the frame)
- K is the exposure multiplier (1 for the base estimate, 2 for a relaxed estimate with twice the predicted drift)
The star trail check mode inverts the problem. Instead of asking how long you can expose, it takes the shutter speed you plan to use and reports how far the stars will smear across the sensor:
Trail (px) = 15.041 * t * cos(D) * FL / (206.265 * p)
The constant 15.041 is the sidereal drift rate of the sky in arcseconds per second, and 206.265 * p / FL is the image scale of your setup in arcseconds per pixel. Pixel displacement is quantitative; whether it is visible depends on optical blur, focus, processing, viewing distance and output size. One-pixel and three-pixel times are comparison references, not universal sharpness thresholds.
Pixel Pitch and Declination Reference
The 500 rule is a rough estimate and can allow visible trailing, particularly when inspecting small pixels or large outputs. The table below shows the pixel pitch of popular cameras and compares the base NPF-based limit against the 500 rule for a 20mm lens at f/2.8. Use the pixel pitch column directly in the calculator if you prefer to enter it by hand.
| Camera | Sensor | MP | Pixel pitch | NPF-based estimate (20mm f/2.8) | 500 rule (20mm) |
|---|---|---|---|---|---|
| Sony a7 III | Full frame | 24 | 6.0 µm | 13.9 s | 25 s |
| Sony a7 IV | Full frame | 33 | 5.1 µm | 12.6 s | 25 s |
| Canon R5 | Full frame | 45 | 4.4 µm | 11.5 s | 25 s |
| Nikon Z8 | Full frame | 45.7 | 4.3 µm | 11.4 s | 25 s |
| Sony a7R V | Full frame | 61 | 3.8 µm | 10.6 s | 25 s |
| Sony a6700 | APS-C | 26 | 3.8 µm | 10.6 s | 16.7 s |
| Fujifilm X-T5 | APS-C | 40.2 | 3.0 µm | 9.4 s | 16.7 s |
| Canon R7 | APS-C Canon | 32.5 | 3.2 µm | 9.7 s | 15.6 s |
| OM System OM-1 | Micro 4/3 | 20.4 | 3.3 µm | 9.9 s | 12.5 s |
Notice that every camera in the table hits its NPF limit well before the 500 rule time runs out. That gap is the reason images that follow the 500 rule often still show short trails when viewed at full size.
Star speed across your frame also depends on where you point the camera. Stars near the celestial equator move fastest, while stars near Polaris barely move. The multipliers below show how much longer you can expose compared with the equator limit. Enter the declination in the calculator’s optional settings, using the declination closest to 0 degrees anywhere in your frame if you shoot wide.
| Target | Approx. declination | Exposure multiplier |
|---|---|---|
| Celestial equator (Orion’s Belt) | 0° | 1.00x |
| Milky Way core (Sagittarius) | 29° | 1.14x |
| Andromeda Galaxy (M31) | 41° | 1.33x |
| Cassiopeia (Schedar) | 56° | 1.79x |
| Big Dipper (Dubhe) | 62° | 2.13x |
| Polaris | 89° | 57x |
Example Problems
Example 1. You shoot a 20mm lens on an APS-C body with a 1.5x crop factor and want the classic 500 rule answer. The effective focal length is 20 * 1.5 = 30mm, so t = 500 / 30 = 16.7 seconds. The nearest standard camera setting below that limit is 15 seconds.
Example 2. You shoot the Milky Way core with a 20mm f/1.8 lens on a 33 MP full frame camera. With an approximate full-frame pixel pitch of 5.12 microns, the base NPF-based limit at the celestial equator is (35 * 1.8 + 30 * 5.12) / 20 = 216.6 / 20 = 10.8 seconds. The core sits at a declination of about −29 degrees, so dividing by cos(29) stretches the limit to about 12.4 seconds. For comparison, the 500 rule would have suggested 25 seconds. Running 25 seconds through the star trail check shows roughly 6 pixels of drift, whose visibility should be checked in a test frame.
Frequently Asked Questions
Why do stars still trail when I follow the 500 rule?
The 500 rule does not account for pixel pitch, aperture, target declination or output size. As the illustrative reference table shows, an NPF-based estimate can be substantially shorter, but neither rule guarantees a sharp image. If your stars are trailing, switch the calculator to NPF mode, or run your current shutter speed through the star trail check to see exactly how many pixels of drift you are accepting.
What declination should I enter if I do not know it?
Leave the field blank or enter 0. That assumes your target sits on the celestial equator, where stars move fastest, so the result is the largest local drift for the entered exposure and setup. If you know your target, use the reference table above or a planetarium app to find its declination and gain extra exposure time. With a wide angle lens the frame spans a large range of declinations, so use the value closest to 0 that appears anywhere in your composition, not the value at the center.
Do these limits apply with a star tracker or when stacking?
A star tracker follows the sky and reduces this drift. Alignment, tracking errors and other conditions still limit tracked exposures. Without a tracker, the standard workaround is stacking: expose each frame at the NPF limit, take 10 to 30 identical frames, and combine them in stacking software. Stacking reduces noise as if you had exposed longer, but stacking does not remove motion blur already recorded within each exposure. Inspect individual test frames before choosing an exposure.
