The 500 rule and NPF rule for sharp stars
Long exposures make stars trail. Learn how to calculate the maximum shutter speed for pinpoint stars with any lens and sensor.
Because the Earth rotates, stars move during long exposures. Exceed a certain time and they become short lines instead of points.
The 500 rule
Divide 500 by your effective focal length:
- 14mm on full frame: 500 ÷ 14 ≈ 35 seconds
- 20mm on full frame: 500 ÷ 20 = 25 seconds
- 16mm on APS-C (24mm equivalent): 500 ÷ 24 ≈ 20 seconds
Why it is not enough
High-resolution sensors like the 61-megapixel A7R V show trailing earlier. Many photographers use 300 instead of 500 for sharper stars.
The NPF rule
The NPF rule considers aperture, pixel size and focal length, giving shorter, more accurate times. PhotoPills calculates it for your camera automatically.
Compensating
Shorter exposures mean less light. Open the aperture wider or raise ISO, then use stacking to reduce noise.
Tracking
A star tracker removes the limit by following the stars, allowing exposures of several minutes.
