The DSLR NPF Rule
If you have ever went out to take photos of the stars at night with your DSLR it is likely you have been told or you have researched and found the 500 rule. Sometimes this rule is called the 600 Rule or the 450 rule. These are simple rules to remember out in the field when you need to quickly calculate the amount of time you need exposure to the night sky to get a good star field without trails.
There was one problem with these rules. As one increased the image size the stars seemed elongated. This is something I have struggled with as I love taking star field pictures whether astro-photography or astroscapes. Images I thought were good always had elongated stars and it was frustrating. Thankfully a man in France found a solution. Before I discuss the NPF rule, lets discuss the 500 Rule.
500 Rule (or 600 or 450)
The 500 rule was a carry over from film photography. it is a simple solution. The time to expose the film to the sky was equal to 500 ÷ focal length in mm. So if a person was using a 50mm lens or a zoom lens to 50mm the calculation would be 500 ÷ 50 and the result would be 10 seconds. If you use a 600 Rule or 450 rule the calculation is the same, just replace 500 with 600 or 450. The rule was the golden rule because there was general limitations to film photography. With the advent and popularization of DSLR cameras suddenly the golden rule was appearing to be outdated. It remains a good standard, but as technology advanced and cameras could capture larger and larger images, the elongated stars became more and more of a problem.
The NPF Rule
The NPF rule was developed by Frédéric Michaud for the Société Astronomique du Havre of France. He explains what NPF means N for aperture, P for Pixel size, F for focal length.The best I can find is that he published his formula for the NPF calculation in 2016. I found an announcement post he created on cloudynights.com. Fred, as he signed his name on cloudynights, also has an official website for the NPF rule. I highly suggest opening it in a web browser that can translate whole websites because it is written in French. In a humorous way Google translates the website by also translating the abbreviation to MFN. So son’t be surprised if NPF becomes MFN when you visit the page and translate it.
The NPF rule is very detailed. It takes into account the actual hardware of the camera you are using. Therefore, it creates a very precise measure of time to create the exposure. I have since come to call this the maximum time which one will capture pin point stars. The purpose of this writing is to help you find the numbers you need to calculate this yourself. Why? There are a few tools out there that will provide you data for many camera models. These can range from a couple hundred camera models to a couple thousand camera models. As cool as these tools are, you don;t need the data for that many camera. You just need the data for your collection of cameras.
Lets look at the simplified formula. This is the basic everyday use formula:
Exposure time = ( (35 x aperture) + (30 x pixel pitch) ) ÷ focal length
Spreadsheet formula =((35*B1)+(30*B2))/(b3)
Formula assumes your respective data for aperture, pixel pitch, and focal length or in cells B1, B2, B3.
If you are new to photography your aperture is your F-Stop. Usually expressed such as f/1.8. The focal length is expressed in mm. The length of your lens. The item you and some long time photographers may not know is the Pixel pitch. This is easily found through searching for your camera model followed by pixel pitch on Google. I use a Canon EOS 40D. So I searched for “Canon EOS 40D pixel pitch on google and found my pixel pitch is 5.7μm. You do not need the lettering. You just need the 5.7 or whatever the pixel pitch is for your camera.
As an example I used the Canon Nifty 50 to shoot a sample picture. I set my aperture at f/2.8. I then used the formula to find that I needed an exposure of 5.38 seconds. Good news is that is the maximum exposure. So I set my camera exposure timer to 5 seconds. The resulting picture is at the top of this post. To compare this to the 500 rule (or 600 or 450) i made a spreadsheet to show all the different results of the calculations:
For some reason I expressed the 450 rule as 400. The calculation for 450 results in 9 seconds.
But wait there is more
That is only the simplified calculation. If you want precision pictures head to the NPF page mentioned above. It gives the simplified result I describe here. Then it goes on to collect data of the exact latitude and longitude you plan to capture pictures and the angle you plan to set your camera. It will then give you the maximum exposure time for each section of the camera grid. This is what it looked like for me with the same settings I used in my spreadsheet calculation.
As seen in the above result I should use 2 seconds to have really crisp stars. For a normal picture this setting would not be the best. However,if you are looking to stack the pictures to create a crisp clean noise free image, then 2 seconds is perfect.
Now for the fun. If you have not looked at Frédéric Michaud tutorial website yet, then I would like to present to you his full NPF calculation for your enjoyment. I honestly don’t know how to read math. I wrote it out for you:
I hope you have learned what you need to know about the NPF formula. I hope it helps you with your stars in your photographs.