Camera Exposure Rule Calculator Formula
Understand the math behind the camera exposure rule calculator. Each variable explained with a worked example.
Formulas Used
500 Rule Exposure
rule_500 = 500 / focal_lengthNPF Rule Exposure
npf_rule = (35 * focal_length + 30 * pixel_pitch) / pow(focal_length, 2) * focal_lengthVariables
| Variable | Description | Default |
|---|---|---|
focal_length | Focal Length(mm) | 50 |
pixel_pitch | Pixel Pitch(µm) | 4.3 |
How It Works
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.
Worked Example
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).
Frequently Asked Questions
What about crop sensor cameras?
Use the effective focal length (actual × crop factor). For a 50 mm lens on APS-C (1.5× crop): 500 / 75 = 6.7 seconds.
How do I take longer exposures?
Use a star tracker (equatorial mount) that rotates to compensate for Earth's rotation. This allows minutes-long exposures.
Why do stars trail?
Earth rotates 15 arcsec/second. At long focal lengths this motion is magnified, causing stars to streak across the sensor.
Ready to run the numbers?
Open Camera Exposure Rule Calculator