Regra NPF (estrelas)
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
About this calculator
The Star Exposure Time Calculator uses the NPF rule to help photographers determine the ideal exposure time for capturing sharp images of stars. The rule takes into account factors such as aperture, sensor pixel pitch, declination, and lens focal length.
The formula behind the calculator is t = (35·aperture + 30·pixel_pitch + 30·decl/foc)/foc, where t is the exposure time, aperture is the lens aperture value, pixel_pitch is the sensor pixel pitch, decl is the object's declination, and foc is the lens focal length. This allows photographers to adjust their settings to avoid star trails.
It's essential to use this calculator in situations where star photography is done with specific equipment, such as cameras with high-sensitivity image sensors and lenses with wide apertures. Correct exposure time helps preserve star sharpness.
When using the calculator, it's crucial to be mindful of measurement units and ensure that the entered values are accurate. Small errors can result in exposures that are too long or too short, compromising image quality.
Frequently asked questions
What is the NPF rule?
The NPF rule is a formula used to calculate exposure time for capturing sharp images of stars, considering aperture, sensor pixel pitch, declination, and focal length.
What units should I use for the values?
Ensure consistent units. Typically, aperture is in f-stops, pixel_pitch in microns, declination in degrees, and focal length in millimeters.
Can I use this calculator for other types of photography?
This calculator is specific to star photography. Different calculators may be needed for other types of photography.
What if I don't know my sensor's pixel pitch?
Check your camera's manual or manufacturer's specifications for this information.
How do I adjust aperture and focal length for ideal exposure?
Adjust these values according to the calculator's recommendations and the specific conditions of your photography session.