NPF/Shutter Time Calculator

This NPF & shutter time calculator uses 5 different techniques of calculating your shutter time. Most people are used to the default "500" rule, but this calculator also has: The Plate Scale calculation, the NPF rule, the simplified NPF rule and the 4 crop rule.

Below the calculator you will find the explanation behind every way of calculating your shutter speed, so you can pick the one that suits you the most.

Sensor size will determine the crop factor of your images.

  • Medium Format: Crop factor of 0.6x
  • Full Frame: Crop factor of 1x
  • APC: Crop factor of 1.5x
  • APC (Canon): Crop factor of 1.6x
  • 3/4: Crop factor of 2x
  • 1 Inch: Crop factor of 2.7x

If you do not know your crop factor go to digicamdb.com find your camera, and search for "crop factor"

Aperture of the lens you are shooting with, this is usually found on the lens itself or the interface, indicated with an F value (example F/1.8)

Focal length of the lens, can be found on the lens/focus rinng, indicated in mm

Pixel size/pitch of an individual pixel on the camera sensor, you can find the pixel size/pitch on digicamdb.com.

Declination is the angular distance in degrees measured from the celestial equator along the meridian through the star. If you are not sure what the declination is, set this to 0 or find your target on stellarium-web.org

Accuracy will be used to calculate the NPF rule, you can select the following:

  • Pin Point: Stars will look like points (very short exposures)
  • Slight trailing: Stars will have some slight trailing (longer exposures but less accurate)
  • Major trailing: Stars will have major trailing (very long exposures, very inaccurate, not recommended)
NPF Calculator Accuracy Example

*Only used for the NPF & 500/300/200 Rule Calculator

Calculated Exposure times:

Plate Scale

0s

Plate Scale**

0s

NPF

0s

Simplified NPF

0s

500 rule

0s

4 crop rule

0s

Plate Scale Calculation

Shutter Speed = (206265 * Pixel Size in mm / Focal Length) / 15

The more technical equations for keeping stars round. Stars on the celestial equator move 15 arc-seconds per time second. So compute the angular size of a pixel to determine the star drift rate. The pixel pitch is called the Plate Scale.

This calculation will result in stars having exactly 1 pixel smear.

** For stars not on the celestial equator, the exposure time will be divided by the cosine of the declination to get the same drift amount in pixels:
Shutter Speed = ((206265 * Pixel Size in mm / Focal Length) / 15) / cos(Declination)

Clark, R. N. (2012, October 20). Nightscape Photography with Digital Cameras. Clarkvision.com. Retrieved August 12, 2022, from https://clarkvision.com/articles/nightscapes/

NPF Rule Calculator

Shutter Speed = Accuracy * (16.856 * Lens Aperture + 0.0997 * Focal Length + 13.713 * Pixel Size) / (Focal Length * cos(Declination))

The NFP rule is a rule that makes it (fairly) easy to calculate the maximum exposure time to take a photo of a starry sky without the stars trailing too much. This rule replaces the old “rule of 500” which gives results that are too uncertain.

Michaud, F. (2020, December 1). La Règle NPF – Société Astronomique du Havre. sahavre.fr. Retrieved August 12, 2022, from https://sahavre.fr/wp/regle-npf-rule/

Simplified NPF Rule Calculator

Shutter Speed = (35 * Lens Aperture + 30 * Pixel Size) / Focal Length

A Simplified version of the original NPF rule that uses a rough formula which uses a declination of 60° instead of 0° and does not take into account the (small) effect of the seeing.

Michaud, F. (2020, December 1). La Règle NPF – Société Astronomique du Havre. sahavre.fr. Retrieved August 12, 2022, from https://sahavre.fr/wp/regle-npf-rule/

The 500/300/200 Rule Calculation

Shutter Speed = 500 / (Sensor Crop Factor * Focal Length)

The 500/300/200 rule is a measurement used to determine the maximum exposure time that can be used in a photograph before star trails appear or before the stars become blurry. If you set the shutter speed for a longer amount of time than what is permitted by this rule, the photographs you take will not have sharp stars.

  • 200 Rule: Pin Point Accuracy
  • 300 Rule: Slight Trailing
  • 500 Rule: Major trailing

The 4 Crop Rule Calculation

Shutter Speed = (4 - Sensor Crop Factor) * 100 / Focal Length

The simplest rule (except for the 500 rule) that is less precise then the NPF rule, but much more precise then the 500 rule.

Michaud, F. (2020, September 18). “4-crop” rule instead of rule of 500 for pin point stars. Cloudy Nights. Retrieved August 12, 2022, from https://www.cloudynights.com/topic/729753-4-crop-rule-instead-of-rule-of-500-for-pin-point-stars/