Astrophotography · Calculator

NPF Rule Calculator

A more accurate alternative to the 500 Rule for keeping stars sharp. Enter your aperture, focal length, and pixel pitch to get the recommended exposure time.

Aperture (f-number)

Focal length (mm)

Pixel pitch (microns)

Recommended max exposure —

Pixel pitch is the size of one pixel on your sensor, in microns. Find it by dividing the sensor width in millimetres by the horizontal resolution in pixels, then multiplying by 1000. Many camera databases list it directly.

How to use the NPF Rule Calculator

You need three pieces of information about your camera and lens:

The aperture you are shooting at, as an f-number (for example f/2.8).
The focal length of your lens in millimetres.
The pixel pitch of your camera's sensor in microns.

Press Calculate to get the recommended maximum exposure time in seconds.

What is the NPF Rule?

The NPF Rule is a more accurate guide than the 500 Rule for the exposure time that keeps stars as sharp points rather than trails. It accounts for your aperture and pixel pitch, not just focal length, so it adapts to your specific camera. The formula is:

(35 x aperture + 30 x pixel pitch) / focal length = exposure time (seconds)

Because it factors in the sensor's pixel pitch, the NPF Rule usually recommends a shorter exposure than the 500 Rule, which gives tighter, sharper stars at the cost of a little light.