Star Shutter Speed Calculator
Stars move across the sky, so a long exposure can turn them into streaks. The 500 rule is a common starting point for the longest time that keeps points of light looking like points. Enter your lens and camera to see the number.
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How it works
Multiply the focal length by the crop factor, then divide 500 by that number. The result is in seconds.
The 500 is a rule of thumb, not a fixed limit. The part of the sky you point at, and how sharp you want the stars to be, both change the result.
Worked example
With a 24 mm lens on a 1.5 crop camera, the 500 rule suggests exposures up to 14 s before stars start to trail.
Questions
Why 500?
The number is a long-standing rule of thumb for stars seen through a wide lens view. Many photographers use it as a starting point, then adjust.
What is an untrailed star?
It is a star that looks like a point of light, not a streak. Longer exposures let stars drift across the frame.
Does this work for the Milky Way?
Use it as a starting point. Faint detail and wider lenses may need different settings, so test a few shots.
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