Photography Rules: 500, 600, NPF, Looney 11, Sunny 16 and more

Photography is a form of art, and as such, one would think there are no rules. But we all know there are things like composition rules that are at work in photography, as well as in paintings.

These “rules” are built on our human brain’s sense of aesthetic, to make the scene more compelling for the viewer, easy to interpret, “balanced”, etc.

But, alongside the “artistic” composition rules (more on this in a different article), there are more fundamental and technical rules helping the photographer to create his artistic vision.

So, what rules are there in photography, other than composition rules, and how will they help you to create better images? 

using the 500 rule
Unless you plan on using a star tracker or similar, then you should use your preferred photography rule (500 rule, 600 rule, etc..) to capture the milky way or deep-sky while avoiding trailing in your images.

How do they help in photography?

Most of the technical rules (more precisely, most of the technical rules of thumb) come from the hay days of film photography when things were more complicated without the direct feedback we have today with digital cameras. 

These rules can be divided into two categories: 

  1. Rules for setting the exposure
  2. Rules for setting the shutter speed

The rules for setting the exposure are there to help you set the proper “average” exposure for the scene, without relying on a light meter.

The rules for the shutter speed are mostly used in astrophotography, and they were created to help the photographer setting the camera to avoid star trailing.

Sunny-16 & Co: The Magic Rules For A Proper Exposure

The most famous of these rules is the Sunny-16, used for having a properly exposed image when photographing outdoors in sunny conditions.

The rules

Here is the list of the most famous exposure rules:

  • Sunny-16: for photographing outdoor in sunny conditions.
  • Snowy-22: for photographing snowy landscapes under sunny conditions.
  • Overcast-8: for photographing outdoor under variable or overcast conditions.
  • Heavy Overcast-5.6: for photographing outdoor with cloudy skies.
  • Sunset-4: for photographing at dusk.
  • Looney-11: for properly expose the full Moon.

While most of the rules are named after “sky” conditions, the key aspect to look for to pick the right one is looking at the shadows. 

Overcast days have very flat light, as the sky acts as a giant softbox. On sunny days, on the other hand, objects cast strong shadows.

Comparing the shadows in sunny and overcast lighting
Comparing the shadows.

How Do These Rules Work?

These empirical rules were originally based on the response and dynamic range of films, but they are also valid in today’s digital world.

The numbers in the rule names indicate the suggested aperture to use in combination with a film sensitivity of ISO 100 and a shutter speed of 1/100th of a second to have a properly exposed image.

Remember: at the time of the film, there was no in-camera image stabilization. Therefore you had to use fast enough shutter speeds.

And this brings to mind another rule of thumb in photography: to avoid camera shake, your shutter speed should be the inverse of the focal length you are using, or faster.

With a 200mm lens, this rule will tell you to use a shutter speed of at least 1/200th of a second.

The Sunny-16 rule states that you can properly expose a bright outdoor scene using an aperture f/16, a shutter speed of 1/100s, and ISO 100.

On a strongly overcast day, the Heavy Overcast Rule tells you to increase your aperture from f/16 to f/5.6, being all the other settings the same.

The Exposure Triangle

But what if you had in your camera a roll of film ISO 400 to finish? What if your lenses cannot be stopped down to f/22 or stopped up to f/4, as required by the snowy-22 and sunset-4 rules?

No problem, just use the exposure triangle, i.e., the unique relationship between film sensitivity, lens aperture, and shutter speed in determining the exposure of the image.

The exposure triangle diagram
The exposure triangle, linking together aperture, shutter speed, and ISO. 

In photography, there are many combinations leading to the same global exposure of the image.

Let’s consider the Sunny-16 rule: the image is properly exposed for f/16, ISO 100, and 1/100s.

The same proper exposure can be obtained using these (and many more) settings combinations:

  • f/11, ISO 100, 1/200s
  • f/16, ISO 200, 1/200s
  • f/16, ISO 50, 1/50s

If you want to know more about how the exposure triangle works, have a look at this article.

A final reminder: while the exposure, i.e., the brightness of the image, is the same for all those combinations of settings, the visual result can change.

Combinations with slow shutter speed can introduce motion blur, and combinations with different apertures will show a different depth of field.

Rules For Astrophotography

If the Sunny-16 and the other rules about setting the exposure right are just anecdotal in today’s digital world, with in-camera accurate light meters, some photography rules are still actual and widely used.

Especially in astrophotography.

The 600-, 500-, 400-Rule and the other N-rules

Due to Earth spinning, almost everything that lives in the night sky above us can be described as a faint source of light in constant movement.

With the notable exception of the Sun, the Moon, and the brightest planets, long exposures are a must for photographing the starry night and its many wonders.

The problem is that stars move during such long exposures, leaving noticeable trails on your images.

If you are doing star trails, all is nice and good, but if you want to take detailed images of the deep-sky, you don’t want the stars and your target to trail.

parking on location and capturing star trails
Star trails are a great way to create interesting night shots.

Without the need to go for a fully grown astrophotography mount, capable star trackers are getting popular and more and more affordable.

Star trackers are lightweight equatorial mounts that allow moving the camera in sync with the stars so that we can take minutes long exposures without trailing stars.

1 hour star tracker timelapse
1 hour long timelapse of an astrophotography session with the Skywatcher Star Adventurer PRO tracking the stars.

This image is the result of photographing the Pacman Nebula for a total of four hours with my SkyWatcher Star Adventurer PRO, using 4 minutes-long exposures.

Pacman Nebula in Cassiopeia
The Pacman Nebula in Cassiopeia. Note the round stars despite exposing the sky with 4 minutes-long exposures.

But what if we don’t have a tracker? What if we only have a classic tripod to mount a camera on?

The solution is keeping your exposure short enough so that stars will not trail noticeably, yet long enough to collect as much light as possible.

One popular way to guesstimate how long an exposure is short enough so that stars do not look like trailing on the image is to use the 500-rule.

The 500 Rule Explained

The 500-rule states that, in order to find your slowest possible shutter speed, you have to divide 500 by the focal length used.

If you are photographing the Milky Way with say, an 18 mm lens, according to the 500-rule you can expose the sky for 500/18=28s. Longer than that and stars will trail in a noticeable way.

That’s not so bad: with a fast wide-angle lens, it seems like you can get away without a tracker. 

van parked with milkway in the sky
A starry landscape from a fixed tripod, stacking 8 images taken with a Sony RX100 Mk ii. 28 mm f/1.8 ISO 6400 15s.

But let’s now try to photograph The Great Orion Nebula with a 200mm lens and see what happens. Now, using the 500-rule once more to calculate our maximum exposure length, we found we can expose the sky for only 500/200=2.5s

That’s short! Comparing this to what you can get using a star tracker is … painful, to put it gently.

Great Orion Nebula
The Great Orion Nebula photographed from a fixed tripod (top) and with the Star Adventurer PRO (bottom).

Nonetheless, using a simple tripod is the first step into astrophotography, and learning how to balance the need for long exposure with the need to avoid trailing stars can let you take nice images.

To help astrophotographers to avoid trailing stars on their images, the 500-rule was derived.

The 500-rule (as well as the other N-rules, such as the 600 or 400-rules) was developed for film photography when the photographer could not see the resulting image and adjust the camera settings accordingly on the fly.

The 600 and 400 Rules Explained

The 600-rule is less stringent, allowing you for longer exposure time, while the 400-rule is more strict and results in a shorter exposure time than both the 600- and 500-rule.

All N-rules have the same form: N / FL, where FL is the focal length and N is a number, typically 600, 500, 400, or 200. 

The 600 rule, for example, reads as 600/FL, where FL is the focal length, and N is chosen depending on how short you want the trails to be: the smaller the N, the shorter the trails

But where do these numbers come from? They were empirically determined by considering the resolution of the film-lens combination. 

Because the size of the light-sensitive particles in high ISO film is often larger than the pixels in digital sensors, digital cameras have higher resolution and the 500-rule falls short.

With today’s high-resolution digital cameras, one should more likely use at least the 400-rule, if not the 200-rule.

The big advantage of using an N-rule is that the math is very simple, and you can easily calculate the exposure time.

If we want to be a bit more accurate, we can modify the 500-rule to qualitatively take into account the optical resolution. 

Crop sensors often have smaller pixels than full-frame sensors and, therefore, have higher optical resolution. 

The higher the resolution the more star trailing is noticeable, thus you have to use shorter exposure. One way to qualitatively account for this is to further divide by the crop factor the time you obtained with the 500-rule

If for a 50mm lens on a full-frame, the 500-rule calculates a maximum exposure time of 10s, for my Olympus micro four-thirds camera I would divide this time by 2 (the crop factor for a MFT camera) and expose for 5s instead. 

But we can do better than this… much better.

The NPF rule

The NPF rule is a different rule for determining the longest exposure time one can use when not tracking the sky, and is much more accurate than the N-rule family.

One reason for this is that stars appear to move in circles around the celestial pole. This means that for a given amount of time, stars near the celestial pole (those with high declination) move less than the stars near the horizon (low declination).

While the N-rules assume all the stars travel the same amount in a given time, the NPF rule takes the declination of the target into account.

The image below shows the comparison between the exposure time predicted by the NPF and 500-rules for stars with a minimum declination of 85º and 10º, respectively.

maximum exposure time with the NPF and 500 rules with photopills app
Comparing the prediction of the maximum exposure time with the NPF and 500 rules for stars with different declination with the PhotoPills app.

If your target is high up in the sky, its declination is high and the NPF rule says you can expose as long as 20s with a 200mm lens on a fixed tripod: that is 10 times longer than what the 500-rule predicts and more than 3-stops worth of light!

You may also have noticed that in the image above, the exposure time was calculated for a Canon Digital Rebel. 

This is because the NPF rule also takes into account things like pixel size and circle of confusion.

My Olympus OM-D EM-5 Mk ii has smaller pixels than those in the Canon Digital Rebel (3.75 and 7.38 micron, respectively): the optical resolution of my Olympus, for a given lens, is, therefore, higher than that of the Canon.

This means that in the time the stars travel the distance of a pixel on the Canon, on my Olympus, they travel for more than 2 pixels, and trailing becomes noticeable. 

The exposure time calculated with the NPF rule for a 200mm lens, on a target with a minimum declination of 85º drops from the 19s of the Canon Digital Rebel to a much shorter 13.6s for my Olympus OM-D EM-5 Mk ii camera.

effect of using sensors resulting in different optical resolution
The effect of using sensors resulting in different optical resolution on the maximum exposure time.

While more accurate than the 500-rule, calculating the maximum exposure time using the NPF rule in the field is obviously more complicated. Or is it?

The Best Calculator For Your Smartphone

We all have a computer in our pockets, always within reach: our smartphone. One of the best uses you can make of it is to have astronomic and photography related apps on it.

PhotoPills, available for iOS and Android smartphones and tablets, is like the landscape and astrophotographer’s swiss army knife.

Aside from featuring augmented reality and being a great planner for landscape and starry landscape photographers, it has plenty of useful calculators, such as the star trail calculator, the NPF calculator, etc.

many tools that are available in AstroPills
The many tools that are available in AstroPills.

If you are interested in astrophotography, this is a neat and useful app to have in your toolbox.


Photography is a form of art built on technical rules, some of which are nowadays less useful, such as the Sunny-16, but many are still popular and improved.

Getting to know them can really improve not only your understanding of how photography works but also on the quality of your images.

About Andrea Minoia

Andrea Minoia works as a researcher in a Belgian university by day and is a keen amateur astrophotographer by night.

He is most interested in deep sky photography with low budget equipment and in helping beginners along their journey under the stars.