Star Trail Calculator

Focal Length

Sensor

Resolution (MP)

Aperture

Latitude: 45\u00B0

500 Rule20.8s

NPF Rule (more accurate)11.6s

RecommendationUse the NPF rule (11.6s) for pixel-level sharpness. The 500 rule (20.8s) is a quick estimate but may show trailing on high-resolution sensors.

Latitude:

What is this?

Stars appear to move across the sky because the Earth rotates. If your shutter is open long enough, stars will streak into trails in your photo. The star trail calculator helps you find the maximum exposure time for pinpoint stars, or plan long exposures to create beautiful circular star trail arcs.

How it works

The Earth rotates 360 degrees in approximately 23 hours and 56 minutes (a sidereal day), which means stars move at about 15 arcseconds per second at the celestial equator. Stars near the celestial poles (Polaris in the north) trace smaller circles and appear to move more slowly. The classic "500 Rule" divides 500 by your effective focal length to estimate the maximum exposure in seconds before trailing is visible. However, this rule was designed for lower-resolution sensors. The more accurate NPF Rule accounts for pixel pitch, focal length, aperture, and declination: it typically produces shorter — and more accurate — exposure limits for modern high-resolution cameras. For intentional star trails, photographers shoot many shorter exposures (typically 30s each) and stack them in software. This avoids noise buildup, allows for gap-free trails, and means a single bad frame does not ruin the entire shot.

Key factors

Focal Length — Longer focal lengths magnify star movement, requiring shorter exposures for pinpoint stars. A 14mm lens can expose 5-10 times longer than a 200mm lens.

Sensor Resolution — Higher-resolution sensors reveal star trailing at shorter exposure times. The 500 Rule is too generous for modern 40MP+ cameras.

Latitude & Declination — Stars near the celestial poles move less per second. Pointing toward Polaris allows longer exposures before trails appear.

Stacking vs Single Exposure — For star trails, stacking many short exposures produces cleaner results than one extremely long exposure, and allows recovery from interruptions.

Pro Tip

Use the NPF Rule instead of the 500 Rule for modern cameras. It accounts for sensor resolution and produces more accurate maximum exposure times, especially with high-megapixel bodies.

Pro Tip

For star trail stacking, keep gaps between frames under 2-3 seconds using a wired remote or intervalometer. Longer gaps create dotted rather than smooth trails.

Pro Tip

Include Polaris (or the South Celestial Pole) in your composition for dramatic concentric circle trails. Stars closer to the pole trace tighter arcs.

⚡Challengebeginner

You are photographing the Milky Way with a 24mm lens on a full-frame 24MP camera. Using the 500 Rule, approximately how long can you expose before stars begin to trail?

Hint: The 500 Rule: maximum seconds = 500 / focal length.

Focal Length

Sensor

Resolution (MP)

Aperture

Latitude: 45\u00B0

500 Rule20.8s

NPF Rule (more accurate)11.6s

RecommendationUse the NPF rule (11.6s) for pixel-level sharpness. The 500 rule (20.8s) is a quick estimate but may show trailing on high-resolution sensors.