500 Rule vs NPF Rule: Which Is Better for Astrophotography?
In this post, I'll go over both the 500 Rule and the NPF Rule. I suggest getting familiar with both rules so that you understand what they're used for and how they differ. All you need to know is included on this page!
You can also learn more about this in my free untracked astrophotography course.
What is the "500 rule" and what is it for?
Night sky photography is all about collecting light. Collecting light is achieved primarily by taking long exposure shots. Of course, if you are only using a tripod and camera to image the night sky, as we are in this course, you'll quickly start seeing star trails if you let your exposure go for too long.
The 500 rule is a quick and easy way to calculate the maximum length of exposure you can use when you image the night sky untracked until you start seeing star trails. All you need to calculate that number is the focal length of your lens.
The focal length of your lens is easy to find, it should be inscribed on the lens itself, for example: "EF-S 24mm." In this case, the focal length of this lens is 24mm. For zoom lenses, the focal length can vary, for example: "EF-S 55-250mm" and so that number will depend on how zoomed in you are with the lens.
Once you know the focal length of your lens, you only need to follow a simple formula to find out how long your exposures should be. The formula slightly differs depending on the type of camera you have (APS-C or Full-Frame) so we'll list both below. We'll use the 135mm lens as an example.
APS-C (Cropped sensor) cameras:
500 / (1.6 * Focal length of the lens) = Maximum exposure time in seconds
500 / (1.6 * 135mm) = Maximum exposure time in seconds
500 / 216mm = 2.31 seconds
Rounded: 2 seconds
Full-Frame cameras:
500 / Focal length of the lens = Maximum exposure time in seconds
500 / 135mm = 3.70 seconds
Rounded: 4 seconds
Note: The crop factor between an APS-C and a full-frame camera depends on the brand.
The crop factor for Canon cameras is x1.6
The crop factor for Nikon and Sony cameras is x1.5
The 500 Rule is the easiest way to know how long your exposures should be, and it works most of the time. But this was created back when cameras were old and had low resolution, so it is now far from perfect as you may see some minimal star trailing depending on the camera you use.
The best way to calculate the longest exposure time free of star trails is the "NPF Rule". You'll want to use that if you're aiming for the best results without a star tracker.
What is the NPF Rule?
The NPF Rule is an equation that will tell us how long our exposures can be when doing untracked astrophotography before seeing star trails. The NPF Rule is more accurate than the 500 Rule.
It was created in 2010 by Frédéric Michaud for the Société Astronomique du Havre. He announced how and why he came up with this equation on this webpage. Most of it is in French, but scroll down far enough and you find an interactive plugin in English (see screenshot below) that allows you to input your camera and lens information to easily find out the total exposure time without having to do any math!
The 500 Rule only takes into consideration the focal length of the lens, and... that's it. The main addition to the NPF Rule is that it also adds the sensor dimensions into the equation. Those dimensions differ depending on the camera you plan to use, and they do affect how long your exposures can be before you start seeing star trails. The 500 Rule is great for beginners as it is a quick and easy way to determine that number, but you are likely to still see slight star trails depending on the camera used.
As a reminder, the general 500 Rule equation was:
500 / Focal length of the lens
Now... the full NPF equation is:
2.6 / t=k*(16.856*N+0.0997*f+13.713*p)/(f*cos(δ))
Ouch.
The good thing? You don't need to learn this equation! You don't even need to care about it at all. Thanks to online calculators, like the one on the Havre Astronomical Society's webpage, finding out the perfect exposure time is easily done in seconds!
Remember Photopills, the app I talk about in my post about my 10 favorite astrophotography apps?
To access it, look for the "Spot Stars" feature in the main Pills tab. Make sure the app knows what camera you are using by first going into My Stuff > Settings and selecting your camera. In our case, the app knows we'll be using the Canon Ra.
Inside the Spot Stars page, you'll see your camera on the top, and four logos:
The left one is for the focal length of the lens, so make sure to tap on that and select the focal length of the lens you plan to use.
Next is the aperture, so be sure to select the f/number you plan to shoot at. We almost always shoot at f/4 to avoid coma on our edges, but with the excellent Rokinon 135mm lens, we can shoot wide open at f/2 without issues!
The third logo is for the minimum declination. This can be left at 0 if you're unsure, which is what we do. It does not have much of an impact and will have a very minimal impact on the result.
Lastly, the fourth logo allows you to switch between two accuracy levels, Default (barely noticeable trails) and Accurate (no trails at all). We suggest leaving this at the default unless you plan on getting your images on large prints later.
Once this is done, both the "NPF Rule" and "500 Rule" slots on the page will update with the recommended exposure time. If you use the same camera and lens each time, you never have to actually worry about changing any of the settings, and it only takes a couple of seconds to check the suggested exposure time for each rule.
As you can see, this definitely makes this rule much less scary, allowing you to completely bypass the math part of it and get straight to imaging!
In my example, the NPF Rule tells me to shoot at a maximum of 1,79 seconds. Usually, you can safely round up this number, so 2 seconds for me.
Some other apps that also have this feature are PlanIt, Pin Point Stars, and DSLRDashboard.
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Which one to use between the 500 Rule and the NPF Rule?
The 500 Rule is the easiest way to know how long your exposures should be, without having to think too much or use a calculator. It works okay some of the time, but with modern sensors you're way better off using the NPF Rule.
So instead of using the 500 Rule, use the NPF Rule!
The NPF rule is much more accurate because it includes the real factors that cause trailing:
focal length
aperture
sensor pixel size
declination (optional advanced factor)
The NPF formula (simplified by rounding the numbers) is:
t=k*(16.9*N+0.1*f+13.7*p)/(f*cos(δ))
Very confusing, yes. But don’t worry, you can use THIS WEBPAGE to calculate the exact number in just a few seconds.
Another way is to use an app like Photopills, which has an NPF calculator as well (Inside the “Spot Stars” page)!
Extra Example:
Let’s say you’re shooting with a 24mm lens at f/2 and a camera that has a pixel size of 4.3µm.
The NPF Rule says to shoot for 8.3 seconds (rounded to 8 seconds). But if you were to use the 500 rule, you would shoot for 20.8 seconds (rounded to 20 or 21 seconds).
That’s a very specific and hardcore difference, but in this case it’s huge! 20 seconds at 24mm will almost always show trailing on modern sensors.
Final Thoughts
I hope you found this post useful, especially if you enjoy doing untracked astrophotography or plan to start soon!
If you want to learn everything about shooting deep-sky objects without a telescope or tracker, be sure to go through my 100% free online course which includes videos, text lessons, downloadables and more! It will teach you how to get an amazing picture of the Andromeda Galaxy from beginning to end, with just a camera, lens, and tripod.
Clear Skies,
Galactic Hunter
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