500 Rule Astrophotography Calculator
Find the longest exposure you can use before stars turn into short trails. Enter your camera sensor, lens focal length, aperture and resolution to get the classic 500 rule and the more accurate NPF rule, plus a table for other focal lengths.
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Used to work out the pixel pitch for the NPF rule.
Overrides the value from megapixels if you know it.
0° (celestial equator) is the worst case; stars near the pole move less.
How to use the 500 Rule Astrophotography Calculator
- Choose your camera sensor or enter its crop factor.
- Enter the real focal length printed on the lens and the aperture you will shoot at.
- Enter the megapixels (or pixel pitch) so the NPF rule can account for your sensor's resolution.
- Optionally enter the declination of the part of the sky you are shooting.
- Set the shutter speed to the suggested value: NPF for sharp stars at full size, 500 rule for web-size images.
500 rule and NPF rule formulas
500 rule: t = 500 ÷ (focal length × crop factor). Stricter versions use 400 or 300 instead of 500.
NPF rule (simplified): t = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel pitch in µm and f the real focal length in mm. Pixel pitch ≈ sensor width (mm) × 1000 ÷ horizontal pixels.
Declination: divide either result by cos(δ) for stars at declination δ.
Example: full frame, 20 mm, f/2.8, 24 MP (pixel pitch 6.0 µm) → NPF = (35 × 2.8 + 30 × 6.0) ÷ 20 = 278 ÷ 20 = 13.9 s, while the 500 rule gives 25 s.
500 rule chart (seconds)
| Focal length | Full frame (1×) | APS-C (1.5×) | Canon APS-C (1.6×) | Micro 4/3 (2×) |
|---|---|---|---|---|
| 10 mm | 50 s | 33 s | 31 s | 25 s |
| 14 mm | 36 s | 24 s | 22 s | 18 s |
| 16 mm | 31 s | 21 s | 20 s | 16 s |
| 20 mm | 25 s | 17 s | 16 s | 13 s |
| 24 mm | 21 s | 14 s | 13 s | 10 s |
| 35 mm | 14 s | 10 s | 9 s | 7 s |
| 50 mm | 10 s | 7 s | 6 s | 5 s |
For pin-sharp stars on a modern 24 MP or higher camera, expect roughly half these times; use the NPF result above.
Frequently asked questions
What is the 500 rule in astrophotography?
Divide 500 by the full-frame equivalent focal length to get the longest exposure in seconds before stars visibly trail. With a 20 mm lens on full frame: 500 ÷ 20 = 25 s. On an APS-C camera (1.5× crop) the same lens gives 500 ÷ 30 ≈ 16.7 s.
What is the NPF rule?
The NPF rule, developed by Frédéric Michaud for the Société Astronomique de France, includes the aperture and pixel pitch. The simplified form is t = (35 × N + 30 × p) ÷ f, with N the f-number, p the pixel pitch in micrometers and f the real focal length in mm. It gives shorter, more accurate limits for modern high-resolution sensors.
Why are my stars still trailing with the 500 rule?
The 500 rule was made for film and small prints. Today's 24 to 60 MP sensors show smaller movements, so many photographers use the 300 or 400 rule or the NPF rule, which can be roughly half the 500 rule time.
Do I use the real or the equivalent focal length?
For the 500 rule, multiply the real focal length by the crop factor (or enter the real focal length and the sensor here). For the NPF rule, use the real focal length; the sensor size is already included through the pixel pitch.
What is declination and why does it matter?
Declination is a star's angle from the celestial equator. Stars near the equator move fastest across the frame; stars near the celestial pole (Polaris) barely move. Dividing by cos(declination) gives longer allowable times when you shoot toward the pole. Leave it at 0° for the safe worst case.
How do I get longer exposures without trails?
Use a wider lens, a faster aperture to gather light quicker, raise the ISO, stack several short exposures, or use a star tracker that rotates the camera with the sky.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.