Astrophotography NPF Rule Calculator
Fill in the values and the answer updates as you type.
How the Astrophotography NPF Rule Calculator works
The NPF rule is (35 × N + 30 × pixel pitch in µm) ÷ focal length, giving a time in seconds. It accounts for aperture and pixel size, so it is stricter than the older 500 ÷ (focal length × crop factor) rule, which suits low-resolution sensors. Aim north or south of the celestial equator and trails get shorter. Use the actual focal length, not an equivalent. Example: A 20 mm f/2.8 lens on a 5.9 µm sensor allows about 13.8 seconds, while the 500 rule suggests 25 seconds.
Inputs and results
The Astrophotography NPF Rule Calculator takes 4 inputs and gives 3 results.
What you enter
- Focal length (mm) — starts at 20.
- Aperture (f-number) (f/) — starts at 2.8.
- Pixel pitch (µm) — starts at 5.9.
- Crop factor (for the 500 rule) — starts at 1.
What you get
- NPF rule exposure (seconds)
- 500 rule exposure (seconds)
- 500 rule vs NPF (×)
Example: Astrophotography NPF Rule Calculator with the starting values
With focal length 20 mm, aperture (f-number) 2.8 f/, pixel pitch 5.9 µm and crop factor (for the 500 rule) 1, the calculator returns:
| NPF rule exposure (seconds) | 13.8 |
|---|---|
| 500 rule exposure (seconds) | 25 |
| 500 rule vs NPF (×) | 1.8 |
Change any value above and the results update straight away.
Astrophotography NPF Rule Calculator FAQ
How do I use the Astrophotography NPF Rule Calculator?
Enter focal length, aperture (f-number), pixel pitch and crop factor (for the 500 rule) in the fields, then read npf rule exposure (seconds), 500 rule exposure (seconds) and 500 rule vs npf (×) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Astrophotography NPF Rule Calculator calculate?
Longest exposure before stars turn into trails. It returns npf rule exposure (seconds), 500 rule exposure (seconds) and 500 rule vs npf (×) from the 4 values you provide.
What values does the Astrophotography NPF Rule Calculator start with?
It opens with focal length 20 mm, aperture (f-number) 2.8 f/, pixel pitch 5.9 µm and crop factor (for the 500 rule) 1 so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Astrophotography NPF Rule Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Astrophotography NPF Rule Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.