Quick Tips: What are the NPF Rule and 500 Rule in night sky photography & astrophotography?
The NPF rule provides a quick method for determining the maximum shutter speed that can be used in your shot without causing star trails when shooting astrophotography and nightscape photography. If you’re familiar with the 500 rule (or 600, or 400 rule), it’s kind of similar but provides more accurate results, especially with today’s higher megapixel cameras. If you’re not familiar with the 500 rule, then don’t worry! We’re going to cover that too.
A key challenge in night sky photography is achieving correct exposure in conditions with little to no light. The exposure triangle—shutter speed, aperture, and ISO—becomes a delicate balancing act when shooting at night. We can increase the ISO, which can help, but it also introduces more noise the higher we go. The apertures of our lens are limited in how wide they can open. But shooting them wide open also increases coma and can distort the stars. This leaves us with shutter speed. Our first instinct might be to go with the slowest possible shutter to allow the most light to reach the sensor. But since this beautiful rock that we call Earth is flying through space and rotating, if we use a shutter speed that’s too slow, we’ll start to notice motion blur in the stars, or as we in the biz call it, star trails. Star trails are great if that’s what you’re going for. But if you’re looking for a nice, pretty shot of a sky full of stars, using a shutter speed that’s too slow will turn those beautiful points of light into streaks or ovals instead of sharp dots. But don’t worry, there’s an easy way to figure this out! In this week’s quick tip, we’re going to look at how to calculate your maximum shutter speed for nightscape photography and how the NPF Rule helps you capture sharp, pinpoint stars.
The 500 Rule…or 600 rule or 400 rule…
For years, the 500 rule was the go-to standard for quickly estimating the maximum shutter speed you could use before stars begin to trail. To find your shutter speed, simply divide 500 by your lens’ focal length. For the sake of making the math easy, let’s say we are using a 20mm lens. In this case, we’d have: 500 ÷ 20 equals 25 seconds. Simple right? This rule originally started as the 600 rule, which worked well in the early days of astrophotography. However, photographers started to notice the stars beginning to elongate into ovals or tiny dashes when producing larger prints. And thus, the 500 rule was born.
As digital camera technology improved beyond 20 and then 30 megapixel sensors, the same issue arose again. The rule was then adjusted to 400, and so on. Additionally, the 500 rule didn’t really suit cameras with smaller sensors. Nightscape photographers experimented further and found that the 250 rule worked well for APS-C sensors, while 200 is better for Micro Four Thirds sensors. These rules use the same straightforward formula: divide the focal length by the number in the rule, and you have your maximum shutter speed. For example, with an APS-C sensor and a 20mm lens, dividing 250 by 20 yields a maximum shutter speed of 12.5 seconds. But these rules aren’t super accurate, so you will definitely want to experiment and really zoom in on those stars to see how they look in your test shots. In my opinion, these rules serve more as a starting point than a strict rule that yields a very specific result. They also don’t take into account that while cameras may have similar sensor sizes, they can hold wildly different megapixels. For example, both the Sony A7S III and A7R V have full-frame sensors of the same size, but the A7S III holds 12.1 megapixels while the A7R V has 61 megapixels. That’s 5 times more pixels packed into the same sensor size. So, as you can probably imagine, a better formula was needed! Which brings us to the NPF Rule…drum roll please!
The NPF Rule
Now, before we dive into the NPF Rule, let me preface it by saying this might sound a bit complicated at first. But stick with me to the end because I have a very simple way to handle it. A few years ago, French photographer and amateur astronomer Frédéric Michaud sought a better solution that worked with the vast array of modern cameras and sensors available today. After lots of hard work, he developed what is now known as the NPF rule. The three letters in its name are actually the variables in his equation instead of an abbreviation. They stand for:
N = Aperture (the letter N is used to represent aperture in the optics world)
P = Pixel pitch (essentially the density of pixels on the sensor)
F = Focal length (from the lens you are using)
Frédéric’s formula considers much more than the old 500 rule, including the camera’s sensor size, the lens aperture, and how densely the pixels are packed on your sensor. However, it is more complex and not something you’d want to be scribbling out by hand in the dark like a mad scientist in the night. Luckily, apps like PhotoPills provide quick calculators where you just select your camera, focal length, and aperture, then the app does the rest. In PhotoPills, you can find it inside the Spot Stars pill. As a bonus, PhotoPills can also account for star declination. More on this in a future article, but it essentially indicates how far north or south a star is from the celestial equator, helping us estimate how fast it seems to move through the sky and form trails.
How does the NPF Rule work?
For those who want to understand how it works or just love math, let’s dive into the NPF rule. The formula looks like this:
Shutter Speed in seconds = [(35 x Aperture) + (30 x pixel pitch)] ÷ focal length
To figure out your camera’s pixel pitch, you’ll need to use a separate formula:
Pixel Pitch = (Physical width of camera’s sensor in mm ÷ number of pixels in width) x 1000 micrometers (µm)
To determine the pixel pitch for my Sony Alpha 1, which has a sensor size of 35.9 x 24 mm and a maximum image resolution of 8640 x 5760 pixels, I would use the formula like this:
4.16 = (35.9 ÷ 8640) x 1000 µm
This shows that the pixel pitch of the Sony Alpha 1 is 4.16 µm. From here, I could then figure out the NPF formula. If we stick with our same 20mm lens from the other formulas, and let’s say we are shooting at an aperture of 2.8, we can find that the maximum shutter speed for this configuration would be 11 seconds. That is a massive difference from the 500 Rule’s answer, which gave us 25 seconds! Here’s what the formula would look like:
11.14 seconds = {(35 x 2.8) + (30 x 4.16)} ÷ 20
NowwNow, depending on what your output is going to be for that shot (social media post, digital, small prints, large print, etc), you may want to bend the rules a little bit. For example, if I know the shot is only going to be used for digital outputs like a post on instagram or a timelapse, then I might increase my shutter from 11 seconds up to 12 or 13. If I think I might print the photo in the future, I’ll stay as close as possible to the NPR Rule’s shutter speed. You’ll want to experiment a bit to see how much you can bend the rule and still achieve results youare okay with. And again, doing math while out on a beautiful night under the stars is probably not something you want to deal with while setting up your shot, so I’d highly recommend downloading PhotoPills. The NPF rule calculator is just one of the many reasons I love that app so much. It’s my go-to for planning shots, scouting locations, and a wide range of other tasks.
Interested in capturing stunning nightscapes and night sky photos? Join my one-on-one online classes, personalized tutoring sessions, or immersive in-person photography workshops, and let me help you unlock your full potential. Whether you’re just starting out or looking to elevate your skills to new heights, I can guide you to create breathtaking images. Check out this link for more info!
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