Max Exposure for Star Trails
Find the longest shutter speed before stars start to trail (500 rule).
About Max Exposure for Star Trails
The 500 rule divides 500 by the effective focal length to give the seconds before Earth's rotation visibly trails stars. Staying under the limit keeps stars as points rather than streaks when you do not want intentional star trails.
Formula
Max Exposure for Star Trails
Max shutter = 500 / (focal x crop factor)
- eff focal
- Focal length after applying the crop factor
Worked example
With the default values below, Max Exposure for Star Trails returns Max Shutter (500 rule) of 13.89. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Focal lengthminimum 1
- 24
- Crop factorminimum 0.5
- 1.5
Results
- Max Shutter (500 rule)
- 13.89 s
- Conservative (35/F)
- 0.97 s
Step by step
- 1Values used
- 2Max Exposure for Star Trails
- 3Max Shutter (500 rule)= 13.89 s
- 4Conservative (35/F)= 0.97 s
Assumptions and limits
- The result is derived only from the 2 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
How is Max Exposure for Star Trails calculated?
Max shutter = 500 / (focal x crop factor) The 500 rule divides 500 by the effective focal length to give the seconds before Earth's rotation visibly trails stars.
Why does Max Exposure for Star Trails matter?
Staying under the limit keeps stars as points rather than streaks when you do not want intentional star trails.
What values do I need to enter?
This calculator takes 2 inputs: Focal length, Crop factor. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.