Star trail length is calculated using Earth's rotation rate (≈0.00418° per second), your focal length, exposure time, sensor crop factor, and the declination of the stars you're photographing. The formula is: Trail (μm) = Focal Length (mm) × Exposure (sec) × 0.0729 × cos(Declination°) ÷ Crop Factor. Divide by pixel size to get the trail in pixels. Stars near the celestial equator produce the longest trails, while stars near the celestial poles produce almost no trails. Use our calculator above to instantly compute this for your setup.

These rules differ in tolerance for star trailing. 400 Rule (400 ÷ focal length) is the most conservative, producing shorter exposures ideal for high-resolution sensors or large prints. 500 Rule is the classic balance, suitable for most cameras and web-sized images. 600 Rule is more lenient, allowing longer exposures at the cost of slightly more visible trails—acceptable for social media or when you need more light. Choose based on your output medium and personal standards. For modern 40+ MP sensors, the 400 Rule or NPF Rule is recommended.

The NPF Rule (named after its creator Frédéric Michaud) is a modern formula that accounts for aperture (N), pixel size (P), and focal length (F): NPF = (35 × N + 30 × P) ÷ F. Unlike the 500 Rule, NPF considers how pixel density affects star trail perception. A 61MP camera with tiny 3.76μm pixels will show trails much sooner than a 24MP camera with 5.93μm pixels—even at the same focal length. The NPF Rule is widely considered the most precise exposure calculator for modern digital astrophotography, especially for pixel-level sharpness.

Declination is the celestial equivalent of latitude. Stars at 0° declination (celestial equator) move fastest across your sensor, producing the longest trails. At 45° declination, trails are about 71% as long (cos 45° ≈ 0.707). At 90° declination (near Polaris or the celestial pole), trail length approaches zero. When shooting near the North Star, you can use much longer exposures. Our calculator includes declination so you can fine-tune your settings based on where in the sky you're pointing.

It depends on your output: < 2 pixels is excellent and virtually invisible even at 100% zoom. 2–4 pixels is good for most purposes; trails are barely noticeable. 4–8 pixels is acceptable for web and social media but visible when pixel-peeping. 8–15 pixels shows obvious trailing—fine for artistic star trail photos but not for pinpoint stars. > 15 pixels means significant trails that will be visible even in small prints. For astrophotography where pinpoint stars are critical, aim for under 3 pixels of trail using the NPF Rule.

Full frame sensors have an advantage: for the same field of view, you use a longer focal length, but the crop factor of 1.0 means the 500 Rule gives you more time. For example, a 16mm lens on APS-C (1.5× crop, 24mm equivalent) gives 500÷(16×1.5) = 20.8 seconds. A 24mm lens on full frame for the same field of view gives 500÷24 = 20.8 seconds—identical in this case. However, full frame sensors typically have larger pixels, which are more forgiving. The real advantage comes from full frame's better noise performance, allowing higher ISOs and shorter exposures.

Pixel size determines how quickly a moving star "spills over" into adjacent pixels. A star trail of 30μm on a sensor with 6μm pixels covers 5 pixels—noticeable but not severe. The same 30μm trail on a sensor with 3.5μm pixels covers 8.6 pixels—much more obvious. This is why high-megapixel cameras (with smaller pixels) demand stricter exposure rules. The NPF Rule directly incorporates pixel size for this reason, making it superior to the pixel-agnostic 500 Rule for modern sensors exceeding 30 megapixels.