I love looking at the night sky and thinking about space and time. It helps me appreciate how we (as a planet) got here, to where we are right now after millions and millions of years. It helps me appreciate how I (as a person) got here, to where I am right now after only 30. How did it come to this and where are we (as a planet) headed? Where am I headed? That's the real question, where am I headed, because my GPS broke and I don't know how to read a map. I only have a pretty good guess. But that's not why this calculator exists, no, the real reason is because I had the absolute pleasure of talking with someone on the Amtrak train, my seatmate, who told me that sometimes when he tries to take an astro-photo the pictures turn out blurry. This tool will hopefully help astrophotographers determine how to capture sharp stars without trailing or blur. It uses two popular calculation methods, the 500 Rule and the NPF Rule, to prevent star trails in your long-exposure night sky photography.
The 500 Rule is a simple, classic formula: divide 500 by your effective focal length (factoring in crop factor). The result is the maximum exposure time in seconds before stars begin to blur into trails. It's quick and works reasonably well for wide-angle lenses, but doesn't account for your camera's sensor resolution or aperture.
The NPF Rule is a more advanced calculation that considers your camera's megapixels, aperture, and sensor size for a more precise result. By accounting for pixel pitch (how densely packed pixels are on your sensor), it provides better recommendations for higher-resolution cameras. NPF typically gives shorter exposure times than the 500 Rule for modern high-res sensors.
Stars appear to move across the sky due to Earth's rotation. Longer exposures let you capture more light, but if the shutter stays open too long, stars will trail instead of remaining as sharp points. These rules give you the sweet spot—maximum light gathering without visible star movement. For high-megapixel cameras, the NPF Rule is generally more accurate since it accounts for the fact that densely-packed pixels show star trails sooner.
Shutter Speed: Longer exposures gather more light but risk star trails. This calculator helps you find the upper limit for your setup.
Aperture (f/stop): A lower f-number (like f/2.8) lets in more light and allows shorter exposures while still capturing enough light. The NPF Rule takes this into account—wider apertures yield longer acceptable exposures.
Focal Length: Longer focal lengths (telephoto) magnify star movement, requiring shorter exposures. Wide-angle lenses (14-24mm) let you use longer exposures for the same stars.
Megapixels & Sensor Size: Higher megapixel cameras with the same sensor size pack pixels more densely, making them more sensitive to star trails. A 45MP full-frame camera needs shorter exposures than a 24MP full-frame for the same lens. The NPF Rule accounts for this; the 500 Rule does not.
ISO: Controls how sensitive your camera sensor is to light. In astrophotography, a higher ISO brightens faint stars and the Milky Way, but it also increases noise, the random grain that can make images look speckled or lose detail in darker regions. The ideal ISO setting depends on your camera's sensor and how much noise it can handle. Modern Full-Frame cameras can shoot at higher ISOs with minimal grain, while smaller sensors (like APS-C or Micro Four Thirds) often need lower ISOs to maintain cleaner results. The goal is to balance sensitivity with image quality, bright enough to reveal the stars but clean enough to preserve detail in the night sky.
Disclaimer: This calculator uses a simplified version of the NPF rule for educational purposes. Actual optimal exposure times may vary depending on your camera's exact sensor dimensions, pixel pitch, atmospheric conditions, and post-processing tolerance for minor star trailing. For precise astrophotography planning, consult dedicated NPF calculators that account for your camera model's sensor data.