What Is The NPF Rule And How To Use It For Brilliant Star Photography?

Today we are going to get a bit technical and take a look at the NPF rule and how to use it to get tack sharp photographs of the stars.

When you look at the stars in the night sky, you see them as sharp pinpoint lights and so when you photograph them, you will want to record them as sharp points. Due to the rotation of the earth, the stars seem to move across the night sky (although the stars have their own movement around the centre of the galaxy) and during very long exposures, we see those star trails recorded in our images.

Why Sharp Stars?

Photographing star trails is a completely different thing and is done by choice, but when you want to photograph sharp point stars or even the Milky Way, you need to expose your camera’s sensor for the correct period of time, so even when you zoom in to the image or print them large, you will not be able to notice any trails in there.

Sharp Stars Require:

1. Perfect focus
2. Perfect exposure

How Do You Get Sharp Stars?

In order to achieve this, you will need to calculate the correct exposure time based on the type of camera (full frame or APS-C or micro four thirds, etc.), lens combination and use that value as your shutter speed in order to avoid star trails.

Note: To record stars as bright beautiful spots, you will need to record as much light as possible as well, but then you also want to avoid the trails created by the stars by controlling the exposure time.

Rules For Calculating Exposure Time: (500, 600 and NPF Rule)

There are various rules out there to calculate the exposure time for capturing pinpoint stars. You may have heard about the classic 500 rule and the 600 rule for star photography and they have been discussed before in our articles and in a few forum topics as well.

1. What is the 500 rule in Photography?
2. So what exactly is the 600 rule in photography?

Depending on whether photographers notice star trails after following the above rules, they have also used the 400 and 200 rules to get the results they are looking for.

On the PhotoPills website, you can calculate the exposure times for various cameras and focal lengths using the Spot Stars Calculator.

However, the above rules can fail by providing larger exposure times than required and this can cause slight star trails – remember, these rules and calculators were devised with the classical 35mm film in mind.

With the recent high-resolution digital cameras, in order to get the best or more accurate results, you need to have a more accurate exposure time and it is necessary to take into account the camera sensor size, resolution, and aperture beside the focal length.

A formula was devised by Frédéric Michaud for the Société Astronòmique du Havre called the NPF rule which is quite complex, where

• N stands for aperture, the letter N is used to represent aperture in optics.
• P for photosites or pixel density or pixel pitch (distance between two pixels)
• F for focal length

So, according to the NPF Rule…

Exposure time or shutter speed in seconds is approximately equal to {(35 x Aperture) + (30 x pixel pitch)} ÷ focal length and this is the simplified NPF formula.

Pixel Pitch is not a straightforward number and you will need to calculate it as follows:

Pixel Pitch P = (Physical width of camera’s sensor ÷ number of pixels in width) x 1000 micrometer (µm)

So, for example, for a Nikon D750 camera,

• The sensor size is 35.9 x 24.0 mm
• Maximum Image size is  6016 x 4016 pixels

So Pixel Pitch P = (35.9 ÷ 6016) x 1000 µm = 5.967 ≈  5.97 µm

So for example,

If you will be using a 24mm f/2.8 lens on a Nikon D750 camera, then using the above NPF rule, the shutter speed will be approximately equal to {(35 x 2.8) + (30 x 5.97)} ÷ 24 in seconds ≈ {98 + 179} ÷ 24 seconds ≈ 11.54 ≈ 12 seconds

If we had calculated the exposure time using the 500 rule, it would have been 500 ÷ 24 = 20.8 ≈ 21 seconds. An image printed really large with this setting would show up some trails instead of point stars.

The NPF rule can be quite a hectic thing to calculate. If you are looking for a free calculator, there is a qDslrDashboard app, where you tap on the “Rule 600” and then it will let you choose the sensor size, input the sensor width and height, aperture value, focal length, image width and height and then it will calculate the shutter speed for you according to the NPF rule.

Also, the calculation does not just stop there if you are looking for the most accurate exposure time.

If you have not thought about this before, the speed of Earth’s rotation at the equator is higher compared to other latitudes. It decreases by the cosine of latitude and in this regards, you will have to take the declination from the celestial equator into account as well when calculating the exposure time for sharp stars.

Why do these factors above need to be taken into account?

Here is a quick read that will be of some help.

Resolution of camera sensor – as the resolution of the sensor increases, star trails become more evident at the pixel level because of the increased pixel density. So, as the resolution increases, the exposure time needs to be decreased to eliminate star trails.

Declination from the celestial equator – Declination is nothing but the angular distance of any point in the sky that is north or south of the celestial equator. At the celestial equator, the speed of earth’s rotation is the highest, causing the most amount of star trails. The further you move away from the equator, the increase in declination which means a decrease in the speed of rotation as discussed above and hence fewer star trails will be seen at the pixel level. The equatorial declination is 0^o at the celestial equator.

To calculate declination accurately, you need to know where you are and at which direction you are pointing the camera at the night sky. The best way to find this is using a smartphone app like PhotoPills or Planit.

You can find the apps here:

1. PhotoPills for iOS and Android
2. Planit for iOS and Android

Leave declination at the default value of zero, if you are not very sure about it.

Now, if you are someone who has the PhotoPills app, then the shutter speed can be calculated according to the detailed NPF rule easily, using the app. Once you open the app, choose the “Spot Stars” pill.

The app takes you to an area where you can choose your camera, input the focal length, aperture, declination and calculate the default exposure time according to NPF and 500 rule.

In the screenshot below, you can notice the difference in shutter speed for the Nikon D750 and D850 cameras because it takes into account the megapixels of your camera as well.

If you do not know the declination for the location where you are, just tap on AR at the bottom of the app screen and point your smartphone towards the direction and angle you are planning to shoot and that should display the declination at the bottom of the screen for you. It also shows the shutter speed for the particular camera, focal length and aperture combination in this screen, so you do not have to manually input the values for calculation.

You can see that the app takes the declination into account when calculating the shutter speed using the NPF rule. For different declination, the shutter speed is different.

In the PhotoPills app, you can see that there is a default setting that may have barely noticeable trails. If you are going for really large prints, like more than 24 inches wide and if you need very high accuracy in point sharp stars, you may choose “Accurate settings” in the app. This will, however, decrease the exposure time by half which you may have to compensate by increasing the iso.

For anyone interested in an advanced astrophotography shutter time calculator, the article here at Lonely Speck goes into various factors on calculating the exposure time for astrophotography.

Don't forget to get a wider perspective on this with our guide on How to Photograph the Milky Way and remember that moon photography is quite different to either!