Shutter speed rule calculator
Pick a rule of thumb, enter your lens and camera details, and get the shutter time to try. The star rules keep stars as points; the 180° rule gives natural motion blur in video.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose the 500 rule, the simplified NPF rule or the 180° video rule.
- For the 500 rule enter the crop factor and the focal length. For NPF enter the actual focal length, the aperture and the pixel pitch in micrometres. For video enter the frame rate.
- Use the shutter time shown as a starting point, then check the result on your screen.
How it works
- 500 rule: 500 ÷ (crop factor × focal length) seconds.
- Simplified NPF: (35 × N + 30 × p) ÷ f seconds, with N the f-number, p the pixel pitch in µm and f the focal length in mm.
- 180° rule: 1 ÷ (2 × fps) seconds.
These are rules of thumb, not guarantees. The sky’s apparent motion also depends on the direction you point and your latitude.
Worked example
A 24 mm lens on a full-frame camera: 500 ÷ 24 ≈ 20.8 s. On a ×1.5 crop camera it is 500 ÷ 36 ≈ 13.9 s. NPF for f/2.8 and 4.3 µm pixels at 24 mm: (98 + 129) ÷ 24 ≈ 9.5 s, which is stricter.
Tips
- On high-resolution sensors the NPF figure is usually the safer choice.
- Zoom in to 100% on a test frame to see whether stars trail.
Frequently asked questions
What is the 500 rule?
A rule of thumb for night sky photos: divide 500 by the full-frame equivalent focal length to get the longest exposure in seconds before stars start to look like short trails. It is a rough guide that predates high-resolution sensors.
Why use the NPF rule?
The NPF rule also takes the aperture and the pixel pitch into account, so it adapts to high-resolution sensors where the 500 rule gives trails. This calculator uses a simplified form, (35 × N + 30 × pixel pitch in µm) ÷ focal length, not the full published formula.
What is the 180 degree shutter rule?
For video, set the exposure time to half the frame time: 1 ÷ (2 × frame rate). At 24 fps that is 1/48 s, usually rounded to 1/50 s on cameras.
Where do I find my pixel pitch?
Divide the sensor width in micrometres by the pixel width of the image. A 36 mm sensor that is 6000 px wide has about 6 µm pixels. Many spec sheets also list it.