Astrophotography Shutter Speed Calculator (500 Rule and NPF Rule)
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Astrophotography Shutter Speed Calculator (500 Rule and NPF Rule) by TheCalcTools
How the Astrophotography Shutter Speed Calculator (500 Rule and NPF Rule) works
The Earth turns, so stars drift across the frame. The traditional 500 rule limits the exposure to 500 ÷ (focal length × crop factor) seconds, made for film and low-resolution prints.
The NPF rule by Frédéric Michaud (Société Astronomique du Havre) also accounts for aperture and pixel size; its simplified form is (35 × N + 30 × pixel pitch) ÷ focal length. It gives pinpoint stars on modern high-resolution sensors.
The formula
Worked example
Using the default values shown in the calculator:
| Focal length (mm) | 20 |
|---|---|
| Crop factor | 1 |
| Aperture (f-number) | 2.8 |
| Pixel pitch (µm) | 5.9 |
- 20 mm on full frame: the 500 rule allows 25.0 s.
- The NPF rule at f/2.8 with 5.9 µm pixels gives 13.8 s.
Result:
| 500 rule (seconds) | 25.0 |
|---|---|
| Stricter 300 rule for high-resolution sensors (seconds) | 15.0 |
| NPF rule, simplified (seconds) | 13.8 |
Frequently asked questions
Why do my stars still trail with the 500 rule?
High-megapixel sensors record trailing that film hid. Use the NPF result, or a star tracker for longer exposures.
Sources and further reading
- National Parks at Night: NPF, the new rule for shooting the sharpest stars
- Wikipedia: Astrophotography
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.