Astrophotography Exposure

Select a sensor format or enter custom sensor width and pixel pitch below.

The 500 Rule

The 500 Rule is a simple guideline for estimating the longest exposure before stars begin to trail. It divides 500 by the effective focal length:

Where:

• t — Maximum exposure time (seconds)
• f — Lens focal length (mm)
• crop factor — Sensor crop factor relative to full-frame (36 mm width)

This rule is easy to remember but tends to overestimate acceptable exposure times, especially with high-resolution sensors where trailing becomes visible at pixel level.

The NPF Rule

The NPF Rule is a more accurate formula developed by Frédéric Michaud for the Société Astronomique du Havre. It accounts for aperture, pixel pitch, and focal length:

Where:

• t — Maximum exposure time (seconds)
• N — Aperture f-number (e.g. 2.8)
• f — Lens focal length (mm)
• p — Pixel pitch (µm)

This formula produces shorter, more conservative exposure times that better match the resolving power of modern high-megapixel sensors.

Declination Adjustment

Stars near the celestial poles rotate through a smaller arc than stars at the equator. The declination factor extends the maximum exposure time for targets away from the equator:

At the celestial equator (0°) the factor is 1 (worst case). Near the poles (±90°) stars barely trail, allowing much longer exposures. In practice, values above ±85° are capped.

Crop Factor

The crop factor is the ratio of a full-frame sensor width (36 mm) to the actual sensor width. It determines the effective field of view at a given focal length:

Smaller sensors have larger crop factors, resulting in a narrower field of view and shorter maximum exposure times.

Practical Tips

• Use the NPF Rule for modern cameras with 20+ megapixel sensors.
• The 500 Rule is sufficient for older or lower-resolution cameras.
• Consider rounding down to the nearest whole second for safety.
• Atmospheric conditions (seeing, turbulence) can mask minor trailing.
• Stacking multiple shorter exposures often produces better results than a single long exposure.