Star Trail Exposure Calculator

What the Star Trail Exposure Calculator Does

This tool tells you the longest shutter speed you can use on a stationary (untracked) camera before the stars in your photo start to blur into short streaks instead of staying sharp points of light. Because the Earth rotates, stars appear to drift across the sky, so any exposure that is too long records that motion as a trail.

It is built for nightscape and astro photographers shooting the Milky Way, constellations, or wide starfields without a star tracker. Enter your lens focal length and your camera's crop factor, and the calculator returns a maximum exposure time in seconds based on the Rule of 500, with stricter alternatives for sharper results.

How It Works: The Rule of 500 Formula

The classic guideline is the Rule of 500. It estimates the maximum exposure in seconds before visible trailing appears:

max exposure (s) = 500 / (focal length in mm x crop factor)

The crop factor converts your real focal length into a full-frame equivalent. Use 1.0 for full-frame, about 1.5 for most APS-C sensors (1.6 for Canon APS-C), and 2.0 for Micro Four Thirds. Wider lenses cover more sky per pixel, so they tolerate longer exposures; longer lenses magnify star motion and need shorter ones.

Many photographers find 500 too generous for modern high-resolution sensors, where trailing shows up under close inspection. The Rule of 300 simply swaps 500 for 300 to give a more conservative, sharper result.

Worked Example With Real Numbers

Suppose you shoot with a 24mm lens on a full-frame camera (crop factor 1.0):

500 / (24 x 1.0) = 500 / 24 = 20.8 seconds, so roughly a 20-second exposure.

Now put the same 24mm lens on an APS-C body with a 1.5 crop factor. The effective focal length becomes 36mm: 500 / (24 x 1.5) = 500 / 36 = 13.9 seconds, so about 14 seconds. Apply the stricter Rule of 300 to that APS-C setup and you get 300 / 36 = 8.3 seconds for noticeably crisper stars.

The NPF Rule for High-Resolution Sensors

The Rule of 500 ignores pixel pitch, so on a 45- or 60-megapixel sensor it can still let stars smear across several pixels. The NPF rule addresses this by factoring in aperture (N), pixel pitch, and focal length, producing a much shorter and more precise exposure time.

A common simplified version is: time = (35 x aperture + 30 x pixel pitch in microns) / focal length. The NPF result is typically far shorter than the Rule of 500, so treat 500 as a quick field estimate and NPF as the choice when you need pixel-level sharpness on a high-megapixel camera.

Tips, Common Mistakes, and Factors That Affect the Result

These rules give a ceiling, not a guarantee. The amount of trailing you actually see depends on where you point the camera and how closely you inspect the image.

• Forgetting the crop factor: entering only the marked focal length on a crop-sensor camera overestimates your safe exposure. Always multiply by the crop factor.
• Direction of view matters: stars near the celestial pole (Polaris in the north) move slowest, while stars near the celestial equator move fastest and trail soonest.
• Pixel peeping vs. final use: a 20-second frame may look fine at web size but show trails at 100% zoom or in a large print.
• Trade-offs: if the calculated time is too short for a bright exposure, open the aperture wider or raise ISO rather than lengthening the shutter.
• Want intentional trails? Do the opposite: use exposures much longer than these limits, or stack many frames, to record long arcing star trails.