Free Camera Exposure Rule Calculator
Calculate the maximum untracked exposure time before stars trail using the NPF rule or simplified 500 rule: t = 500 / (focal_length).
mm
µm
500 Rule Exposure
10.0 s
NPF Rule Exposure37.6 s
500 Rule Exposure vs Focal Length
Astrophotography Exposure Rules
500 Rule (simple)
t = 500 / f seconds (f = focal length in mm)
Stars will begin to trail after this time on an untracked mount with a full-frame sensor.
NPF Rule (more accurate)
Accounts for pixel pitch and is better for modern high-resolution sensors. A shorter exposure may be needed for sharp pinpoint stars.
Example Calculation
50 mm lens with 4.3 µm pixel pitch.
- 01500 Rule: 500 / 50 = 10 seconds
- 02Stars trail after about 10 seconds at 50 mm.
- 03For crop sensors, divide by the crop factor (e.g., 1.5).