Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
NPF max exposure (sec)
9.5
500 rule (sec)
20.8
Effective focal length (mm)
24
500 rule over-exposure (sec)
11.4
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter focal length (mm).
- 2Enter crop factor.
- 3Enter aperture (f/).
- 4Enter pixel pitch (µm).
- 5Enter declination (°).
- 6Read your npf max exposure (sec) on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Compare the classic 500 rule against the stricter NPF rule for your focal length, aperture, and sensor pixel pitch.
Worked example
Using the values the calculator loads with:
Inputs
- Focal length: 24 mm
- Crop factor: 1
- Aperture (f/): 2.8
- Pixel pitch: 4.3 µm
- Declination: 0 °
Results
- NPF max exposure (sec): 9.5
- 500 rule (sec): 20.8
- Effective focal length (mm): 24
- 500 rule over-exposure (sec): 11.4
What each field means
Inputs
- Focal length (mm)
- The focal length used in the calculation, measured in mm. Starts at 24 mm so you have a working example on load.
- Crop factor
- The crop factor used in the calculation. Starts at 1 so you have a working example on load. Accepted range: 0.5–3.
- Aperture (f/)
- The aperture (f/) used in the calculation. Starts at 2.8 so you have a working example on load.
- Pixel pitch (µm)
- The pixel pitch used in the calculation, measured in µm. Starts at 4.3 µm so you have a working example on load.
- Declination (°)
- The declination used in the calculation, measured in °. Starts at 0 ° so you have a working example on load. Accepted range: -89–89 °.
Results
- NPF max exposure (sec)
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 500 rule (sec)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Effective focal length (mm)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 500 rule over-exposure (sec)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the astrophotography exposure calculator work?
Compare the classic 500 rule against the stricter NPF rule for your focal length, aperture, and sensor pixel pitch.
What do I need to enter?
5 values: focal length, crop factor, aperture (f/), pixel pitch, and declination. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the npf max exposure (sec) result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Astrophotography Exposure Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/astrophotography-exposure
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/astrophotography-exposure" target="_blank" rel="noopener">Astrophotography Exposure Calculator — RevenueLab</a> (2026).</p>
Source: [Astrophotography Exposure Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/astrophotography-exposure) (2026).