Star Trail Exposure Calculator

Work out star trail exposure instantly with clear inputs, formula shown and shareable results.

About Star Trail Exposure Calculator

The 500 rule sets the maximum exposure at 500 divided by the equivalent focal length, and dividing further by the cosine of declination allows for slower apparent motion near the poles. Stars move 15 degrees an hour at the celestial equator but almost not at all near Polaris, which is why northern shots tolerate much longer exposures.

Formulas

500 rule

seconds = 500 / (focal length x crop factor)

Declination allowance

seconds = 500 rule / cos(declination)

Worked example

With the default values below, Star Trail Exposure Calculator returns Longest exposure before trailing of 24.06. Change any input above and every figure here is recalculated in your browser.

Inputs used

Focal lengthminimum 1 mm: 24 mm
Crop factorminimum 0.1: 1
Target declinationaccepts -89–89 deg: 30 deg

Results

Longest exposure before trailing: 24.06 s
Plain 500 rule figure: 20.83 s
Full frame equivalent focal length: 24.0 mm
Star movement in 30 seconds: 0.1085 deg

Step by step

1Values used
2500 rule
3Declination allowance
4Longest exposure before trailing= 24.06 s
5Plain 500 rule figure= 20.83 s
6Full frame equivalent focal length= 24.0 mm
7Star movement in 30 seconds= 0.1085 deg

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 4 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

How is the Star Trail Exposure figure worked out?

seconds = 500 / (focal length x crop factor). The 500 rule sets the maximum exposure at 500 divided by the equivalent focal length, and dividing further by the cosine of declination allows for slower apparent motion near the poles.

Why does this matter?

Stars move 15 degrees an hour at the celestial equator but almost not at all near Polaris, which is why northern shots tolerate much longer exposures.

What do I need to enter?

Enter focal length, crop factor, target declination. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.