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Exposure Time For Astrophotography Calculator

Work out exposure time for astrophotography instantly with clear inputs, formula shown and shareable results.

About Exposure Time For Astrophotography Calculator

The 500 rule caps untracked exposure at 500 divided by full-frame equivalent focal length - 20.8 seconds for a 24 mm lens. Stars near the celestial equator trail fastest, so dividing by cos(declination) extends the limit towards the poles: at 20 degrees declination it becomes 22.2 seconds. High-resolution sensors often need the stricter 300 rule.

Formulas

500 rule

seconds = 500 / (focal length x crop factor)

Declination correction

seconds = 500 / (focal length x crop) / cos(declination)

Worked example

With the default values below, Exposure Time For Astrophotography Calculator returns Maximum Exposure Before Star Trailing of 22.17. Change any input above and every figure here is recalculated in your browser.

Inputs used

Lens focal lengthminimum 1 mm
24 mm
Sensor crop factorminimum 0.1
1
Target declinationaccepts -89–89 deg
20 deg

Results

Maximum Exposure Before Star Trailing
22.17 s
Plain 500 Rule Time
20.83 s
Full-Frame Equivalent Focal Length
24.0 mm
Stricter 300 Rule Time
12.50 s

Step by step

  1. 1Values used
  2. 2500 rule
  3. 3Declination correction
  4. 4Maximum Exposure Before Star Trailing= 22.17 s
  5. 5Plain 500 Rule Time= 20.83 s
  6. 6Full-Frame Equivalent Focal Length= 24.0 mm
  7. 7Stricter 300 Rule Time= 12.50 s

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does declination matter?

Because stars near the pole trace smaller circles and move more slowly across the frame, so they tolerate longer exposures.

Why do some people use the 300 rule?

Because modern high-pixel-density sensors resolve trailing that the 500 rule allowed. Pixel-level tests are stricter still.

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