# Using the NPF Rule for Photographing Night Skies

If you look into the night sky long enough, you will see the stars move. Because it is dark at night, you will need a longer shutter speed to capture the beauty of the stars and constellations. But don’t expose too long, or they will turn into trails.

When you photograph the night sky, it is wise to take the rotation of the Earth into account. Using exposures that take too long will lead to motion blur. The stars will turn into small trails. The longer we expose the image, the longer the trail will become. Sometimes we want to do this on purpose, exposing images for hours. Star trail photography is a great thing to do, and it may lead to amazing photos.

But there are times when you don’t want the stars to have motion blur. In those situations you need to take the rotation speed of the Earth into account. For those situations you need to calculate the maximum shutter speed that is possible, to keep the stars the small twinkling lights we see in the night sky.

## The Rule of 600

Because we know the amount of rotation in 24 hours, we can easily calculate the distance a star will move each second. This leads to the Rule of 600. By dividing the number 600 by the focal length of the lens you are using, you will end up with the maximum amount of seconds an exposure may last. That’s easy to remember, and easy to use.

The Rule of 600 originates from the days of analogue photography. That is why the focal length has to be a 35mm equivalent. If you are using a crop sensor, just multiply the focal length by the crop factor. Still, not every image with a shutter speed that is calculated by the Rule of 600, will produce real stars. There is something not right with this rule.

## The Rule of 500, or Even 400

Nowadays, our digital sensors have more resolution than analogue film. It means, motion blur will be visible much sooner compared to analogue film. That is why the Rule of 600 is often changed into the Rule of 500, or even the Rule of 400. It compensates the increased resolution up to a certain point. Still, it is not easy to get the exact maximum shutter speed. Especially because the resolution of digital sensors is getting larger with almost every new camera. That is why you have to take resolution into account, and for that you can use the NPF rule.

## The NPF Rule

The NPF rule originates from Frédéric Michaud from the Société Astronòmique du Havre. It is a complex rule that takes sensor resolution into account. The NPF stands for

• N = aperture (it’s the official notification of aperture in optics),
• P = pixel density, the distance between the pixels on the sensor, also called pixel pitch,
• F = focal length.

With these variables you can calculate the maximum shutter speed in seconds by using the following formula:

T[sec] = ((35 x N) + (30 x P))/F

If you want to use this rule, you need to know the pixel density of the sensor first. This can be calculated by dividing the width of the sensor by the amount of pixels, multiplied by 1000 micrometer (µm)

I use the Canon EOS 5D mark IV, which has a 30mp sensor. The resolution is 6720 x 4480 pixels on a 36 x 24mm sensor. The pixel density is:

P = (36/6720) x 1000 µm = 5.36 µm

If I use a 16mm lens with an aperture of f/2.8, we can use the NPF rule to calculate the maximum shutter speed for photographing the night sky without motion blur.

(35 x 2.8) + (30 x 5.36))/16

=

(98 + 160.8) / 16

=

16.2 seconds

For comparison, if we calculate the shutter speed with the Rule of 600, we end up with 37.5 seconds, more than twice as long. With the Rule of 500 we end up with 31,2 seconds and 25 seconds with the Rule of 400.

To get an idea how the resolution will influence the maximum shutter speed, we can also use the NPF rule for the 61mp full frame Sony a7R IV with 16mm focal length and f/2.8. The pixel density for this camera is 3.75 µm. By using the NPF rule we end up with a maximum shutter speed of 13.1 seconds. By using a sensor that has twice the amount of pixels, the difference is 3 seconds. That may not sound a lot, but it is the difference between points of light, or small ovals.

## Take the Declination Into Account

There is one thing I did not mention. The stars in the night sky are not moving at the same speed. If a star is further from the celestial pole, the angular speed is still the same, but the star has to travel a larger distance.

The location of a star in the night sky has a declination. It is the distance of a star measured from the celestial equator. The closer the star is to the celestial equator, the larger the distance it will move. A star at the celestial equator has a declination of 0°. The Pole star in the Northern Hemisphere, which is almost exactly at the celestial pole, has a declination of almost 90°.

## Use Photopills

The NPF rule is quite different from the Rule of 600, 500, or 400. It is much more complicated and difficult to use when you are out into the field. Of course you need to know the pixel density of your camera, but even then it might be easier to use the Rule of 400 and go on the save side.

You can do the math at home, and write down the numbers. But it is much easier to use a good app. It will do the math for you, and you can also take declination into account.

Photopills is probably the best app to use. It has a “Spot Stars” option in the Pill menu. There you can choose your camera, focal length and aperture, and the maximum shutter speed is calculated for you. You can compare it to the Rule of 500, which number is also given. There is a default calculation, and a accurate calculation, that can be used if you will make large prints. The following screenshots show how it’s done.

You can also enter the declination, if you know the number. That might be a bit more difficult to find out, although there might be apps that can give you the declination of a star. But don’t worry, just use the augmented reality function of Photopills. Point the cross hair at the star in the center of your composition, and the maximum shutter speed is corrected for the declination. Just see the next screenshots to see how much difference it makes, pointing almost North and almost South.

## Use the NPF Rule for Better Night Sky Images

I did not know the NPF rule, but it was pointed out by two helpful readers of my recent article about the preparations for the Perseid meteor shower, which also makes use of Photopills. Check it out if you like. But I looked into the NPF rule as mentioned in the comments, and as from now I will keep on using the NPF rule. Thank you very much.

How about you? Did you know about this rule, and how to use the app Photopills to get an accurate shutter speed? Of do you use another app, or another way of determine the right shutter speed. Please share your knowledge with us. I am looking forward to your comments.

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Nando Harmsen is a Dutch photographer that is specialized in wedding and landscape photography. With his roots in the analog photo age he gained an extensive knowledge about photography techniques and equipment, and shares this through his personal blog and many workshops.

Is that all you have to say?

Nando,
Thanks for a very well written article. I found it extremely informative.
This is a subject that I admittedly do not have much experience in.
Thank you for broadening my horizons (no pun intended).

You're welcome. Thanks for commenting

Always been a bit skeptical about that article, seems just an overcomplication and now that I look at it more carefully it seems to have some real issues.
First of all: why would aperture matter at all while calculating maximum exposure time to avoid trails? How is the aperture size related in any way to the length of the projected star trail on the sensor? Diffraction maybe? I've tried to read the original article but it's in French and the translator makes a bit of a mess.
Anyway, I've tried plugging in some numbers: Nikon D750 with 24mm f/1.4 gives 9.5s. Fine, seems a bit short to me but whatever... then I've tried plugging in 24mm f/2.8: 11.5s. Wut? That's a lot of difference for a change that pretty much has zero impact on trail size in the actual world. Tried pluggin in my D90 with 24mm f/2.8, suggested time: 11s.
Are you telling me that I'd have the same trails on a D90 with a 24mm at 2.8 as on a Fullframe? Cuz that's definitely not what happens in the real world. Also if you try to plug in some stupid data like 24mm f/12 on a D750 the suggested exposure time becomes a ridiculous 25s which is more than the 20s suggested by the "500 rule"!

Second: I still don't understand why bothering with pixel pitch of the camera since it doesn't impact the actual lenght of the trail.
It doesn't really matter if you are using a 12mpx or 40mpx camera, the trailing on the sensor is the same, and so is the size in the final image.
This is an example I posted a while ago on an italian forum discussing the same thing, the middle image simply is the "real star projection" on the sensor, the left image is simulating a low mpx camera while the right image is simulating a sensor with 4x pixel count.
As you can see with the same exposure time (same trail) the trailing is actually SMALLER in the final image on the high mpx camera, and that's even more prominent in the real world where the star (which because of diffraction and other optical limitations will never be smaller than 1 pixel) always bleeds over multiple pixels.
And to clarify, before somebody jumps on and says that the left one looks more round, plase note that for simplicity I have used solid colors to show affected pixels, in reality the bottom right pixel in the low mpx simulation would have been pretty much uneffected resulting in an elongated shape just like the high mpx simulation.

I think it just MIGHT be useful if you really wanted to have stars as small as possible, but if you are really so concerned about that just buy a tracker.

My suggestion is: do some test. Shoot at different exposure times, you decide what is the acceptable compromise of sharpness and noise, especially since it mostly depends way more on the final destination of the photo rather than the pixel size... it all falls apart if you are shooting to post on Instagram anyway.