The rule is part of a larger family of empirical rules that have been developed, mostly in the days of film photography. These help photographers quickly setup their cameras for the most common situations.
With film, in fact, the pressure was on to get it right in camera as much as possible. You could only see the results once you’ve developed the film.
In the digital era, however, we can check the results in real time. We can afford to be a bit sloppy and to adjust settings on the fly. Nonetheless, by learning those rules we can work better and faster even when shooting digitally.
Notable examples of such empirical rules are the sunny-16, overcast-8 and sunset-4 rules. These deal with setting a proper exposure in specific daylight conditions. And for your night time photography ,the looney-11 rule will help you to properly expose the Moon.
The 500 Rule Explained
The idea behind the 500 rule is to provide you with an easy-to-remember formula to freeze the stars movement and to get star trails free images.
Of course, if your goal is to do star trails, you should ignore this rule. Otherwise you need to use it so that you can capture clean images of the Milky Way and the starry sky in general.
But why are you getting star trails in the first place? That’s because the Earth is rotating on itself once every day. This rotation creates the rather fast 15º/hour apparent motion of stars.
In astrophotography, we make a big deal out of freezing the stars. To be rigorous, the length (in mm) of a star trail on your image will depend on the field of view of your camera-lens combination, the sensor size, the image resolution, the exposure time and the star angular speed and declination.
While this is not rocket science, the formula that links all those variables together is not the simplest one to remember and to use in the field.
If you are an occasional star shooter, knowing how to use the much simpler 500 rule will let you capture reasonably trail free starry skies.
The rule reads like this: SS = 500 / (FL * CF)
SS is the shutter speed in seconds, FL the focal length expressed in mm and CF is your sensor’s crop factor, i.e., the ratio between the size of a full frame sensor and yours.
Common crop factors, CF, for different types of camera are:
- 1 for full frame cameras;
- 1.6 (1.5) for Canon (Nikon) APS-C cameras;
- 2 for micro four thirds cameras;
- 2.7 or higher for compact cameras with a 1″-type sensor or smaller.
As far as I know, while it can be loosely related to image resolution and field of view at a specified focal length, the use of the number “500” doesn’t have an actual meaning. It is an arbitrary constant chosen so that the rule will work decently in most cases.
The 500 rule In Action
Let’s consider my Olympus OM-D EM-5 Mk II micro four thirds camera, which has a crop factor equal to 2. Now let’s see how this rule works.
With a 50mm lens, for example, the 500 rule will tell me the exposure time for which the stars will not trail is:
SS= 500/(50*2)= 5 seconds.
With a full frame camera with the same lens, the exposure time will be:
SS=500/(50*1)= 10 seconds.
The image below shows how much stars are trailing when I use a shutter speed of 3 minutes with my 50mm lens on my Olympus. This exposure time is much longer than that the rule suggests.
By using the 500 rule you can even get decent results when photographing bright deep sky objects such M42 (The Great Orion Nebula) using a telephoto lenses, particularly if you allow stars to trail a bit more than usual.
While the level of details will not be impressive, it will be a very satisfying experience and I would recommend you give it a try.
The 400-, 600- and NPF Rules
There are two common variants of the 500 rule; the 400- and 600-Rule, respectively.
In the 400-Rule, the number 500 in the formula above is replaced by 400. This results in an even shorter exposure time. With the 600-rule, the number 600 is used instead, resulting in slightly slower shutter speeds, i.e., longer exposure times.
If you want better results, you should switch to other rules, such as the NPF rule. These are more accurate and rigorous than the 500 rule.
Nowadays, you can find many star trail calculators online, as well as smartphone apps such PhotoPills. These can help you choose the best exposure time for your camera-lens combination.
Improving Your Night Sky Photography With the 500 Rule and Image Stacking
The first thing to do to improve your images is to use RAW instead of JPEG. This will ensure you have the maximum flexibility when you need to edit your images.
If you are not tracking the sky movement with a tracking head, the stars will always move across the sensor. The light you can collect for each pixel depends solely on how long the star will stay over the same pixel.
You may be tempted to bump up your ISO, but this will only increase image noise, with no extra benefits.
Allowing stars to trail a bit more by using longer exposures will not help either, as the time a star will excite the same pixel will not change. You will just record a trail.
The solution is called image stacking.
By taking many photos at a fairly low ISO, each exposed according to the 500 rule, you can then combine (stack) them later on and this will dramatically improve the amount of details in the final image.
The process involves masking and aligning the sky among all the exposures, but software like Deep Sky Stacker, Sequator (windows, free) and Starry Landscape Stacker (Mac OS X, commercial) will make the whole procedure faster.
Below you can see the comparison between a single raw from my Sony RX100 Mk ii pocketable compact camera (1″ sensor type) …
… and the final image, obtained but staking 8 different exposures. You can see how stacking has boosted details and enriched the sky.
You can read more about image stacking in this article about editing techniques for astrophotography.
For the Curious Minds: the Science Behind the 500 Rule
The idea behind the 500 rule is to provide an easy way to guesstimate the longest exposure time for which the stars’ movement is not noticeable.
The sky rotates 360 degrees in 24 hours, or 0.0042 arc degrees per second. A full frame camera with a 24mm lens, will have an horizontal view of about a 73.7 degree.
Let’s assume we have a 24 Mpx sensor (6000px X 4000px). Those 73.7 degrees are projected onto 6000 horizontal pixels, giving 81.4 pixels per degree.
Assuming a 24mm lens, the “rule of 500” gives you an exposure time of about 21 seconds (500/24).
In 21 seconds the sky will move about 0.09 degrees (0.0042*21).
For our 24 Mpx full frame camera with a 24mm lens, 0.1 degrees translates to 7.3 pixels (81.4*0.1).
Those 7.3 pixels represent the maximum acceptable movement blur before point-like stars will turn into star trails. But is this movement really acceptable?
We are used to looking at our images on a computer screen. If you blow up your full resolution image to 100%, you will see that the star is not a dot.
But what about prints?
It turns out that if you print your image in 30x45cm format, those 7 pixels will make up a trail on your print only 0.5mm long!
If you find yourself struggling to capture that starry night sky photography, using the 500 rule (together with image stacking) will dramatically improve your results, at no extra cost.
And with the holiday season upon us, this rule is something you should remember when you find yourself under that dark, exotic sky with the Milky Way Galaxy shining strong through the night.
For more low light photography tips check out our twilight portrait or night street photography posts! Enthusiastic astrophotographers should check out our SkyWatcher Star Adventurer Review.