What is the NPF rule & How to Use it in Astrophotography

Taking pictures of the night sky requires using long shutter speeds. However, using a long shutter speed may result in star trails. Here comes the NPF rule, a formula developed to assist photographers in achieving sharp star points in their photos.

What is the NPF Rule?

The NPF rule, devised by French photographer and amateur astronomer Frédéric Michaud, is a complex formula used to determine the optimal shutter speed for capturing sharp stars in astrophotography. This rule is a modern solution to the challenge of getting pinpoint stars in night sky photography.

NPF Rule Formula

(35 x Aperture + 30 x Pixel Pitch) / Focal Length = Shutter Speed (in seconds)

The key components of the NPF rule formula are:

• N (Aperture): The aperture value of the lens, represented by the f-number (e.g. f/2.8, f/4, etc.)
• P (Pixel Pitch): The distance between individual pixels on the camera’s image sensor, measured in microns (μm). This can be calculated as: (Sensor Width in mm / Sensor Resolution in Pixels) x 1000 = Pixel Pitch in μm
• F (Focal Length): The focal length of the lens being used, measured in millimeters (mm)

By plugging these variables into the formula, you can calculate the maximum shutter speed in seconds to use in order to capture sharp stars without star trailing.

The NPF rule is considered more accurate than the simpler 500/600 rule, as it takes into account the camera’s sensor resolution and pixel density in addition to the aperture and focal length.

Use Photopills

Instead of doing the math I prefer using the Photopills app to get pinpoint stars. Photopills has incorporated the NPF rule into its “Spot Stars” calculator. Thus making it much easier to determine the optimal shutter speed without having to manually calculate the complex formula. You simply need to select their camera model, input the focal length of the lens, and enter the desired aperture. The app will calculate the maximum shutter speed for the NPF rule and the 500 rule as well. It even allows you to use the augmented reality feature to measure the declination of the stars in your frame, further refining the calculation.

Here I calculate the maximum exposure time for my Fujifilm X-T20 with the Samyang 12mm f/2.0 lens. The NPF Rule suggests an exposure time of about 15 seconds while the oldest and less accurate 500 Rule suggests almost double the time!

Conclusion

In conclusion, mastering the NPF rule is essential for photographers looking to elevate their astrophotography skills. By understanding this formula and utilizing tools like PhotoPills, you can confidently set your camera parameters to capture breathtaking images of the stars and the Milky Way.

Here is a collection with some of my best astrophotography photos if you want to have a look!