Star Trail Calculator

Shooting Time31m

Exposure30m

Display Mode

Exposure / frame (sec)

Shutter speed for each individual frame

Frames

Total number of photos to stack

Gap (sec)

Delay between frames for buffer write

Focal length

Sensor

Resolution (MP)

Aperture

Position45°

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 Stars Move

The Earth rotates 360° in approximately 23 hours and 56 minutes (a sidereal day), so stars move at about 15 arcseconds per second at the celestial equator. Stars near the poles (Polaris in the north) trace smaller circles and appear to move more slowly.

The 500 Rule vs. NPF Rule

The classic "500 Rule" divides 500 by your effective focal length to estimate maximum exposure before trailing is visible — but it was designed for lower-resolution sensors. The NPF Rule accounts for pixel pitch, focal length, aperture, and declination, producing shorter and more accurate limits for modern high-resolution cameras.

Stacking for Star Trails

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 doesn't 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.

Shooting Time31m

Exposure30m

Display Mode

Exposure / frame (sec)

Shutter speed for each individual frame

Frames

Total number of photos to stack

Gap (sec)

Delay between frames for buffer write

Focal length

Sensor

Resolution (MP)

Aperture

Position45°

What is this?

How Stars Move

The 500 Rule vs. NPF Rule

Stacking for Star Trails

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.