500 Rule vs NPF Rule: Shutter Speed for Astrophotography

Sample Milky Way landscape photo
NIKON Z six + 20mm f/1. 6 @ 20mm, ISO 1600, 20 seconds, f/2. 0

When you’re photographing the night sky, it can be a serious problem to pick the right camera configurations. Shutter speed in particular is really a difficult one, forcing you to definitely fight between capturing enough light or capturing sharpened stars. Two popular rules aim to help – the particular 500 rule and NPF rule – but just how do they work in practice? Any better than the other? This article explains everything you need to know.

Table of Contents

Balancing Star Movement plus Noise

Let me start by explaining the issue at hand, and why it’s not as easy to solve as it may seem.

With common (daytime) landscape photography, you generally can use any shutter speed you want and get sharp pictures, assuming you’re on a stable tripod and absolutely nothing in the scene is moving. For example , the landscape photograph below is shot with 60 seconds and is plenty sharp:

Sharp landscape photo at 60 second exposure
NIKON D780 + VR 100-400mm f/4. 5-6. 3E @ 380mm, ISO 100, 60 seconds, f/8. 0

However , that’s not possible along with Milky Way photography. Due to the Earth’s rotation, the superstars move surprisingly fast over the night sky. It can be tough to see that movement with your eyes, but your camera will get it. Beyond about 30 mere seconds of exposure, you’ll get noticeable blur in the celebrities, even with an ultra-wide lens.

The image beneath, shot at 133 seconds, shows how this obnubilate can get out of control:

Blurry star trails in Milky Way photo at 2 minutes of exposure
NIKON Z7 + NIKKOR Z 14-30mm f/4 T @ 30mm, ISO 1600, 133 seconds, f/4. 0

Actually at seemingly safe shutter speeds such as 20 or even 25 seconds, there will be a few blur when you zoom into the photo. In fact , with an usual wide-angle lens, you won’t completely eliminate star motion until you’re at shutter speeds as short since 5 or 10 mere seconds!

Here’s a demonstration of that. The pictures below are extreme crops associated with astrophotography images – the first captured at 8 mere seconds, and the second at twenty seconds. I used a 14mm ultra-wide lens both in cases:

100% crop of Milky Way with zero star trails taken at 14mm and 8 seconds
NIKON Z7 @ 14mm plus 8 seconds
100% crop of Milky Way photo with small star trails at 14mm and 20 seconds
NIKON Z7 @ 14mm and 20 mere seconds

In case you look closely (or click on to see larger), you can tell that there’s more star movement in the image used at 20 seconds, while the image at 8 seconds has essentially no movement blur. However , that’s not the biggest difference between the two images. More importantly, the photograph taken at 20 mere seconds captures more than twice as much light as the other photograph, resulting in a better signal-to-noise proportion. As a result, it looks solution overall, with less sound and fewer discolored -pixels.

So , exactly what would you say is the optimum balance between motion blur and noise? Is it preferable to get pinpoint stars also at the expense of noise, or is a longer shutter speed preferable – probably even longer than in the particular demonstration above? It’s a dilemma.

The perspective is this: Every photographer should make a decision about their particular “preferred balance” between noise and star movement – and I’ll explain more about this decision below. Nevertheless , once you’ve figured out your personal preferred balance, it’s probable to recreate it flawlessly every time you take a Milky Way photo, no matter the other factors at hand (like your own focal length or the path you’re facing).

That’s where the 500 guideline and NPF rule get play. In theory, these rules make it easy to achieve exactly the same preferred balance every time a person take a Milky Way picture. However , in practice, both guidelines have their own flaws.

To make it easier to understand those flaws, let’s have a look at the different factors that impact motion blur in astrophotography.

Elements Influencing Motion Blur

There are 3 major factors which influence how much motion blur you will capture when photographing the particular stars (assuming a stable tripod and no tracking head). Individuals are your shutter swiftness, focal length, and the path you’re facing.

I’ve already demonstrated how shutter speed can affect the particular motion blur in your stars, so let’s take a look at the other two factors: focal duration and the direction you’re dealing with.

Focal size matters for an obvious cause: As you zoom in, a person magnify everything in your picture, including motion blur. (Using a crop sensor digital camera – or cropping your own photo in post-processing – does the same thing. ) For example , the following photo is used at 24mm and 20 seconds, and while it has some star movement, it’s difficult to notice at web quality:

Wide angle image of the night sky with minimal trails because of the wide focal length
NIKON Z7 + NIKKOR Z 24-70mm f/4 S @ 24mm, ISO 6400, 20 secs, f/4. 0

By comparison, I required the following photo on the same evening at 67mm, all other configurations identical:

Telephoto image of the stars at night with 20 seconds of exposure, showing star trails from focal length
NIKON Z7 + NIKKOR Unces 24-70mm f/4 S @ 67mm, ISO 6400, 20 seconds, f/4. 0

If you’re on a desktop, I recommend clicking on possibly of these photos to view them larger and scroll together. It should be pretty clear the second image has larger star trails. If it’s not clear, or you’re viewing it on a smaller screen, here’s a 100% harvest from both photos (the “before” at 24mm and the “after” at 67mm):

Along with your focal length, another important factor is the direction you face. Most photographers know that the stars are usually rotating more slowly across the North Star (or the equivalent “South celestial pole” region if you’re in the Southern Hemisphere). And, in turn, the quickest stars moving across the night time sky are those along the celestial equator, which is the region directly between the North and South celestial poles.

Here’s a comparison between a photograph taken toward the North Star, and then closer towards the celestial equator, to show how star movement modifications depending on your composition. Both these are 120 second exposures:

Minimal star trailing when pointing toward the North Star
NIKON Z6 + 20mm f/1. 6 @ 20mm, ISO 800, 120 seconds, f/2. two, facing the North Star
Example of faster star movement when changing composition
NIKON Z6 + 20mm f/1. 8 @ 20mm, INTERNATIONALE ORGANISATION FÜR STANDARDISIERUNG 800, 120 seconds, f/2. 2, facing closer to the particular celestial equator

The best shutter swiftness is going to be shorter when your stars are close to the celestial equator. The technical term to get a star’s distance from the celestial equator is its “declination, ” which is measured in degrees.

Along with those three factors, there are a few other variables that are not quite as important, but nevertheless matter: your camera’s -pixel size, lens quality, diffraction at your chosen aperture, and focus precision. These can all of the be lumped into an adjustable I call “smallest theoretical star prior to everything else. ” I’m proud of this title because it really rolls off the tongue.

A good way to imagine the smallest theoretical star variable is this: If you miss focus slightly, the stars are all going to become a bit larger in your image. As a result, if they move a few micrometers during your exposure, it’s not going to give you quite since obvious of a star trail, compared to a smaller star moving the same distance. Basically, such as this:

Star movement comparison between small versus large star

So , a bit actually, if your stars are bigger/blurrier in the first place, you can get away with a longer shutter speed before you notice the motion.   Not really that I’m suggesting a person miss focus or use a low resolution camera sensor just because of this effect. You will lose more sharpness plus image quality than you will get from doing so. However , should you be already using a lower quality imaging system, it’s going to cause enough blur it could hide some of the superstar movement, meaning it’s better to use a slightly longer shutter speed.   So , the “smallest theoretical star” variable still matters (and even factors into the NPF rule’s equation).

That’s enough background information. Let’s compare the particular 500 rule and NPF rule to see how properly they work for Milky Method photography, and which shutter speed you should actually make use of.

The particular 500 Rule

By far the less complicated of the two popular guidelines for astrophotography is the five hundred rule. It recommends that your shutter speed is equal to 500 ÷ Equivalent Central Length.

So , if your full-frame equivalent focal length is 20mm, the 500 rule would suggest which you use a shutter speed associated with 500 ÷ 20 = 25 seconds. If you’re using a 50mm lens instead, it might recommend using a 10 2nd shutter speed (500 ÷ 50).

The advantage of the 500 rule is that it’s easy to remember, plus it’ll get you in the right ballpark for your Milky Way shutter speed. That’s likely why it’s become this type of popular tool among photographers who are first learning astrophotography.

The biggest disadvantage with the 500 rule is that it doesn’t take into account the direction you are facing (nor any of the elements like pixel pitch or blur from diffraction). The particular formula only ever spits out a different shutter velocity when you change focal length, which doesn’t account for all of the real-world factors that apply.

The other major drawback of the 500 guideline is that it’s too lax. In almost every case, irrespective of your composition, you’ll have more blur than ideal when using the 500 rule. This particular issue is easy to fix by using the “400 rule” or “300 rule” instead (the same formula, but with 400 or three hundred rather than 500). However , this particular doesn’t fix the issue of the particular direction you’re facing, therefore it’s a bit like putting a bandaid on a garden hose.

That said, if you like the idea of simplicity, this guideline isn’t worthless. I personally use the “300 rule” version once i know that my composition includes stars along the celestial collar (again, the fastest-moving stars in the night sky). What this means is I’m at 20 seconds of exposure with our 14mm lens and 15 seconds with my 20mm zoom lens when those people fastest-moving stars are in our composition .

Of course , the stars along the celestial equator won’t be in all your astrophotography images, yet it’s actually quite common that they will, especially if you’re utilizing an ultra-wide lens. Case in point: The constellation Orion directly intersects with the celestial equator, and Orion is not terribly faraway from the core of the Milky Way (it’s a bit “up and to the right” from the core if you’re in the North Hemisphere). So , the five hundred rule – or, at least, the 300 rule edition of it – still has some value.

Image Averaging Milky Way Final
Used with the classic 500 principle: a 20mm lens along with a 25 second shutter quickness. Even at web quality, you can likely see some star trailing at the top correct, but it’s not terrible (click to see larger). However , I had been pointing fairly close to the Northern Star here. Composing towards the celestial equator could have made the problem worse.

The particular NPF Rule

A more complex formulation for calculating shutter rate at night is called the NPF rule. Here’s the method:

NPF Rule Formula

  • t = Recommended shutter speed
  • e = Multiplication factor
  • In = F-number
  • f = Lens focal length (millimeters)
  • p = Pixel pitch (micrometers)
  • δ = Minimal declination

The key is to memorize this as soon as possible. Once you do, all it takes is some quick mental math to get sharp photos each time.

Ok, I’m kidding! This calculation is built into a number of astrophotography apps directly, like PhotoPills , Pin Point Stars , and a few others. You’ll have to input some of the variables on your own, but once you do, the particular app will tell you the optimal shutter speed without any calculations necessary on your end.

Here’s how it looks in PhotoPills, for example:

PhotoPills Screenshot of NPF Rule Spot Stars

Unlike the 500 principle, the NPF rule considers the direction you’re facing (AKA “minimum declination” within the formula), as well as your pixel pitch and the diffraction from your chosen aperture.

Except… something seems wrong. Within the screenshot above, the NPF rule says to use roughly an 8 second shutter speed with the Nikon Z7 and a 14mm lens. In case you recall the demonstration from the start of this article, that’s exactly what Used to do – and it was very clear that the 20 second shutter speed had a better balance of image quality by comparison.

The thing we are missing here is the multiplication element “k” at the start of the NPF equation. The 8 2nd recommendation is what happens when k is set to 1, but that is not always what you’ll would like.   In fact , the professional photographer who created the NPF rule, Frédéric Michaud , recommended the multiplication factor anywhere from Nited kingdom = 1 to K = 3. A value of 1 gives you total determine stars at the expense of noise; a value of several means you’re tripling the shutter speed (thus making use of 24 rather than 8 mere seconds in this case), giving you more motion blur but substantially less noise.

This is what I was talking about while i mentioned finding your own “preferred balance. ” I personally prefer a K value of about 2 . 5 or 3, meaning that the dimmest stars within my photos are going to be no more than 3 times as long as they are tall (and the larger, brighter stars inside my photos will have even less blur than that). In contrast, PhotoPills has a “Barely visible trails” option (click the particular button that says “accurate” to switch it) which sets K equal to 2, one more perfectly reasonable preference.

It’s not a massive distinction – PhotoPills’ 2× setting recommends a shutter rate of about 15 seconds, while our 2 . 5× preference results in 20 seconds instead – but it’s still important to figure out what balance you prefer best. Then, you can reproduce this balance 100% of the time in the future simply by multiplying the conventional NPF rule by your favored factor.

Nevertheless , the NPF rule furthermore isn’t perfect, even if you’re doubling or tripling the recommendation. Aside from the mild annoyance of opening an application every time you do Milky Way photography, the calculation does not take into account another variable that will matters here: blur from lens quality.   In other words, the NPF rule uses a hypothetical “perfect” lens which is sharper at f/1. 4 than, say, f/4 (because of increasing diffraction in f/4).   But very few lenses, if any kind of, are like this in the real-world.   The result is that you simply can skew the computation by inputting an ultra-large aperture like f/1. four, especially with a camera that has very small pixels.   For example , with the 61-megapixel Sony A7r IV at 20mm plus f/1. 4, the NPF rule suggests a shutter speed of just a few. 85 seconds! Even multiplying by my preferred “k factor” of 2 . 5× still gives you less than ten seconds, when real-world situations easily allow 12 or 15 seconds instead.

So , despite the NPF rule’s clever inclusion of the path you face and the dimension of your pixels, it nevertheless has some flaws. If you use this as a guide rather than as being a guarantee, you’ll have better luck in the field.

Milky Way taken with modified NPF rule for sharp stars
Used at 20mm and a 15 second shutter speed, along with esentially zero star trailing (even if you click to see larger). This is about second . 8× the NPF rule’s strict recommendation.

What Shutter Speed Should You Use?

Given the particular drawbacks of both the 500 rule and the NPF guideline, it can be tempting to just disregard both of them and make use of trial and error instead. And, frankly, that’s not a bad choice.

To me, the final goal is to get the dimmer stars in your photo to become two or three times as long as they are tall (with the exact factor getting your “preferred balance” We keep talking about; for me, it’s a bit more than 2 . 5×). At that point, the larger, brighter superstars in your photo will only end up being slightly elongated, and you’ll get an excellent balance of noise versus star trailing.

The severe crop below is an example of what I personally aim for. Notice how the dimmer stars are about 2 . 5× as long as they are tall, while the best star looks pretty spherical:

Extreme crop to show optimal length of blur in stars
Taken at 14mm and 20 seconds, not quite facing the celestial equator

The five hundred rule and NPF rule can guide you to the shutter speeds that give you this particular result, but they may still recommend something that’s a bit off from optimal. So , no matter what rule you use, I recommend zooming in on your Milky Method photos when you’re during a call and making sure that they appear something like the image above.

I personally use 20 seconds by default with my 14mm lens, and sometimes as much as 25 seconds. With my 20mm lens, that’s 15 and 20 seconds instead. I typically don’t alter this shutter speed based on my camera or aperture, but I do try to find out which direction I’m facing relative to the celestial equator to help make my decision. However , I’m also a bit more aggressive in general about using lengthier shutter speeds than some astrophotographers are, so I encourage you to test it yourself prior to using these exact values.

Eliminating Star Trails Completely

There are two ways to get zero star trailing in any way in your photos, without an extreme amount of noise: star trackers and image stacking.

The first method demands you to get a specialized tracking head that follows the particular movement of the stars. These types of allow you to use arbitrarily lengthy shutter speeds without cloudy the stars. For instance, I actually took the photo below at more than 14 minutes of exposure on a tracking mind, and the stars are perfect pinpoints because the tracker followed them so well:

Blurred Foreground Star Tracker
NIKON Z7 + NIKKOR Z 24-70mm f/4 S @ 24mm, INTERNATIONALE ORGANISATION FÜR STANDARDISIERUNG 64, 860 seconds, f/4. 0

Of course , the ground is fuzzy now, so I took a separate image and blended all of them together:

Sample photo from Nikon Z7
Two-photo blend using a tracking mind for the stars

That method is effective when your foreground has a sharp edge, like a mountain, yet runs into issues for complicated foregrounds like trees. When this occurs, the better option is to do “image averaging” instead. Basically, you take a series of photos at a relatively short shutter speed like 5 or 10 seconds, combined with a higher ISO. With specialized astrophotography software ( Constellation-filled Landscape Stacker for Mac and Sequator for Windows), you can turn and align the superstars in the image without revolving the foreground, then average together your images to reduce noise.

I’ve composed a full article about the method here if you want more info. But the result is that you can get determine stars with image quality equivalent to using a multi-minute exposure. That’s what I did right here with 14 images, each taken at 10 seconds of exposure:

Star Stacking Final Image
NIKON Z6 + NIKKOR Z 14-30mm f/4 T @ 17. 5mm; typical of 14 images each at ISO 6400, ten seconds, f/4. 0

And you can view the level of detail (and insufficient star trailing) in the crop:

Crop of image stack for high image quality photos of the night sky
Crop of the 14-photo blend using image averaging

With this method, it helps to use a shutter speed no longer than the strictest version of the NPF principle. Even in the over-the-top illustration I gave earlier of the Sony A7r IV with the f/1. 4 lens, it is better safe than my apologies; if the NPF rule indicates no more than 4 seconds of exposure, that’s what you should make use of. (If you don’t have an NPF calculator handy, just work with a shutter speed that seems excessively short, like 5-8 seconds. ) Then, compensate for the individually short exposures by taking 30 or forty images to get an comparative exposure of several minutes long. Blend in your stacking software for great results.

Just like the others, although, this not always a perfect technique. It isn’t fun to consider dozens of photos to combine in a potentially time-consuming procedure later. But for photographers which see the “blur versus noise” dilemma and proudly shout “Neither! ” – picture averaging or a tracking head are your options.

Conclusion

It’s amazing what sort of simple topic like choosing your shutter speed intended for Milky Way photography may lead down all these rabbit holes. At the end of the day (literally), your decision might boil down to a simple choice between 15 seconds, 25 secs, or somewhere in between – and you could even capture a variation of each, in less time than it takes to read this article!

But I do think it’s important to understand these basic principles, including the simplifications that both 500 rule and NPF rule make. You may get good photos most of the time just by choosing a formula’s recommendation, but in difficult situations, it’s critical to get more background knowledge. Even for standard astrophotography, wouldn’t you rather understand what’s going on below the surface, so you can feel more confident about your settings? I know I would.

Spencer-Cox-2020-201-22-12-Composite-Neowise
NIKON Z7 + 105mm f/2. 8; 53 images aligned and averaged; each @ ISO 16000, 3 seconds, f/2. 6

Ideally this article clarified everything you had been wondering about. If you have any questions about the 500 rule, NPF rule, or just astrophotography in general, let me know in the feedback below!

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