What is the 500 Rule?

The 500 Rule states that the maximum exposure time before stars visibly trail is 500 divided by the equivalent focal length (focal length × crop factor). It is a rough approximation that works for wide lenses on full-frame; the NPF Rule is more accurate for modern high-resolution sensors.

What is the NPF Rule?

The NPF Rule is a more precise formula: (35 × aperture + 30 × pixel pitch) ÷ equivalent focal length. It accounts for sensor resolution and aperture. Pixel pitch is derived from sensor size and megapixel count. The NPF Rule consistently gives shorter, safer exposure times than the 500 Rule on high-resolution cameras.

How many degrees do stars move per hour?

Earth rotates 360° in approximately 23 hours 56 minutes (a sidereal day), so stars move at about 15° per hour, or 0.25° per minute. A 1-second exposure at a 50mm equivalent lens will show trails of roughly 0.004° — imperceptible to the eye.

What is stacking and why does it reduce noise?

Stacking combines multiple shorter exposures by averaging or adding them. Random read noise averages toward zero while the actual signal from stars accumulates, improving the signal-to-noise ratio. Stacking also prevents amp glow and hot pixels that build up in very long single exposures.

What gap should I set on the interval timer?

Aim for the shortest gap your camera supports — typically 1 to 2 seconds. Longer gaps create visible dark breaks between trails when stacked in "lighten" mode. This calculator suggests a 1-second gap but you can adjust based on your camera model.

Does the calculator account for polar alignment or latitude?

No. The arc length calculation assumes stars near the celestial equator, where apparent motion is fastest (one degree per four minutes). Stars near the poles move in smaller circles and trail less per unit time. For precise polar trail calculations, use the declination-adjusted formula.

Star Trail Exposure Calculator

Name: Star Trail Exposure Calculator
Availability: InStock
Author: Nham Vu

Enter focal length, sensor size, and desired trail arc to get the single-frame exposure time and frame count needed for star trail stacking.

Camera & Lens Settings

Sensor Format

Focal Length (mm)

Aperture (f-stop)

Sensor Resolution (MP)

Used by NPF Rule for pixel pitch.

Desired Trail Arc (°)

15° = 1 hour, 90° = 6 hours, 360° = full circle.

Point-star Limit Rule

500 Rule NPF Rule

NPF is more accurate for high-res sensors.

Fill in your settings and click Calculate to see results.

Summary

Enter focal length, sensor size, and desired trail arc to get the single-frame exposure time and frame count needed for star trail stacking.

How it works

Select your sensor format to set the crop factor.
Enter the focal length of your lens in millimeters.
Enter the aperture (f-stop) and ISO you plan to shoot at.
Choose whether to use the 500 Rule or the more precise NPF Rule for the point-star limit.
Set the desired trail arc length in degrees (360° = full circle, 15° = one hour of rotation).
Read the maximum single-frame exposure, required frame count, total integration time, and an interval timing suggestion.

Use cases

Plan a star trail shoot and avoid guessing how many frames to capture.
Compare how different focal lengths change trail length per frame.
Decide between a single long exposure and a stacked sequence.
Calculate interval timer settings for a star trail time-lapse.
Understand how crop factor affects the apparent trail length on sensor.
Estimate battery and storage requirements before a night shoot.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu