The 500 Rule and NPF Rule are standard guidelines used by landscape and night-sky photographers to calculate the longest shutter speed possible before star trails appear in an image. Because the Earth rotates on its axis, pointing a stationary camera on a tripod at the night sky causes pinprick stars to blur into streaks if the shutter remains open too long.

The classic 500 Rule estimates this shutter speed by dividing 500 by your effective focal length (lens focal length multiplied by your camera sensor crop factor). This tool computes the formula directly in your web browser, allowing you to quickly determine your exposure threshold before setting up your night shots.

### Worked Numeric Example Imagine you are photographing the Milky Way using a 24mm wide-angle lens mounted on a full-frame camera (crop factor of 1.0): 1. Rule Constant (val1): Enter 500 as your baseline astronomical constant. 2. Sensor Coefficient (val2): Enter 1 for a 35mm full-frame sensor (or the reciprocal crop multiplier). 3. Focal Length / Factor (val3): Enter 24 for your 24mm lens.

The calculator multiplies 500 by 1 and divides by 24, returning a Calculated Result of 20.83 seconds. To ensure pin-sharp stars without trailing at the corners of your frame, you would round down and set your camera shutter to 20 seconds. If using an APS-C crop sensor (1.5x crop factor, effective focal length of 36mm), dividing 500 by 36 yields 13.89 seconds, meaning a 13-second exposure is recommended.

### How Your Calculations Are Processed All calculations run entirely in your local browser using client-side JavaScript. Your numeric parameters are evaluated in memory on your device without sending any data to remote servers or saving any information to an external database.