500 Rule Calculator
Calculate 500 rule from the published formulas and tables for the craft.
Loading calculator…
What this tool does
Calculate 500 rule from the published formulas and tables for the craft. The 500 rule is a film-era approximation and is generous on modern high-resolution sensors. The NPF rule (Frédéric Michaud) accounts for aperture, pixel pitch and declination, and is stricter; stars near the celestial equator (declination 0°) trail fastest.
How to use the 500 Rule Calculator
- Enter or select focal length (mm, actual lens).
- Enter or select sensor crop factor.
- Enter or select aperture (f-number).
- Enter or select pixel pitch (micrometres).
- Enter or select star declination (degrees, 0 = celestial equator).
- Read the calculated result; change any measurement to compare alternatives.
Formula
500 rule: max seconds = 500 ÷ (focal × crop); NPF rule: max seconds = (35 × f-number + 30 × pixel pitch µm) ÷ (focal × cos(declination))
- focal
- Focal length (mm, actual lens)
- crop
- Sensor crop factor
- aperture
- Aperture (f-number)
- pitch
- Pixel pitch (micrometres)
- declination
- Star declination (degrees, 0 = celestial equator)
The 500 rule is a film-era approximation and is generous on modern high-resolution sensors. The NPF rule (Frédéric Michaud) accounts for aperture, pixel pitch and declination, and is stricter; stars near the celestial equator (declination 0°) trail fastest.
Worked example
For 500 rule calculator, the following measurements illustrate the exact method: Focal length (mm, actual lens): 24; Sensor crop factor: 1.5; Aperture (f-number): 2.8; Pixel pitch (micrometres): 4; Star declination (degrees, 0 = celestial equator): 0.
Inputs
- Focal length (mm, actual lens)24
- Sensor crop factor1.5
- Aperture (f-number)2.8
- Pixel pitch (micrometres)4
- Star declination (degrees, 0 = celestial equator)0
Result
- 500 rule maximum exposure (seconds)13.89
- 600 rule maximum exposure (seconds)16.67
- NPF rule maximum exposure (seconds)9.08
- Effective focal length (mm)36
Results explained
- 500 rule maximum exposure (seconds)
- 500 rule maximum exposure (seconds) from the formula above. The 500 rule is a film-era approximation and is generous on modern high-resolution sensors. The NPF rule (Frédéric Michaud) accounts for aperture, pixel pitch and declination, and is stricter; stars near the celestial equator (declination 0°) trail fastest.
- 600 rule maximum exposure (seconds)
- 600 rule maximum exposure (seconds) from the formula above. The 500 rule is a film-era approximation and is generous on modern high-resolution sensors. The NPF rule (Frédéric Michaud) accounts for aperture, pixel pitch and declination, and is stricter; stars near the celestial equator (declination 0°) trail fastest.
- NPF rule maximum exposure (seconds)
- NPF rule maximum exposure (seconds) from the formula above. The 500 rule is a film-era approximation and is generous on modern high-resolution sensors. The NPF rule (Frédéric Michaud) accounts for aperture, pixel pitch and declination, and is stricter; stars near the celestial equator (declination 0°) trail fastest.
- Effective focal length (mm)
- Effective focal length (mm) from the formula above. The 500 rule is a film-era approximation and is generous on modern high-resolution sensors. The NPF rule (Frédéric Michaud) accounts for aperture, pixel pitch and declination, and is stricter; stars near the celestial equator (declination 0°) trail fastest.
Frequently asked questions
Divide 500 by the effective focal length (actual focal × crop factor) to get the longest exposure in seconds before stars visibly trail as points. At 24 mm on a 1.5× crop sensor: 500 ÷ 36 ≈ 13.9 s.
The NPF rule refines the 500 rule with your aperture, your sensor's pixel pitch and the stars' declination: seconds = (35 × f-number + 30 × pixel pitch in µm) ÷ (focal length × cos(declination)). It usually allows less time than the 500 rule on high-megapixel bodies, because it is tuned for critically sharp stars at 100% zoom.
Stars wheel around the celestial pole, so stars near the celestial equator sweep the longest path per second and trail first, while stars near the pole barely move. The NPF formula divides by cos(declination), allowing longer exposures for high-declination targets.
Divide the sensor width in millimetres by the image width in pixels and multiply by 1,000. A 24 MP APS-C body (23.5 mm wide, 6,000 px) has a pitch of about 3.9 µm. Many spec sites list pitch directly; 4 µm is a good default for modern 24 MP crop sensors.
Use NPF if you will inspect or print large, and the 500 rule for quick social-media-sized shots where slight trailing is invisible. When in doubt, take a test frame at the NPF time and zoom in — sensor, lens and standards of sharpness are personal.