One of the biggest challenges for beginners when getting started with astrophotography is taking photos with clearly defined round stars.
Indeed, it is fair to say that “Freeze“The stars are a little obsessed with those in the astrophotography community. If you look up at the sky from the ground, you will notice in your pictures that the night sky appears to be moving. Trying to take a starry sky image on a fixed tripod with long exposure often results in star tracking.
Before we dive into the 500 or 600 rule, it is important to know that we are making certain assumptions about the kit you are using: mainly that you are more of a beginner than an experienced astrophotographer.
If you’re using an entry-level DSLR, the 500 rule will be especially useful for you. However, if you are using a high resolution sensor greater than 30MP then you should try the 600 rule or even look at the NPF rule.
The rule only applies if you are using a stationary tripod (most avid astrophotographers probably will). If you’ve invested in a tracking mount, the 500 rule won’t help you any further.
What are star trails?
The entire reason for using the 500/600 rule lies in star trails.
In astrophotography, the purpose is to freeze the stars as clearly and accurately as possible in order to get a crisp and clear picture of fat round stars.
What they sound like, star trails are when the stars moved during your exposure and you left lines rather than dots. We’re not against star trail photography – you can get some stunning shots this way, but it’s frustrating when these aren’t the pictures you want to capture.
Help is at hand, however! If your mission is to get a clear, crisp picture of the Milky Way, instead of just clicking and hoping you get it right, the 500 rule can avoid a lot of trial and error and preparation before taking a photograph.
The science behind the 500 rule
The simple job of the 500 rule is to help photographers get a rough idea of the slowest possible shutter speed before the stars’ movement becomes apparent.
When you look at the stars, it’s easy to forget that the world is always moving. The sky actually rotates at 0.0042 arc degrees per second – or in plain language a full 360 degrees in 24 hours.
If you usually access your photos on a computer screen, you might not mind that the dots aren’t as stationary as you hoped they would be. Of course, you’ll likely only notice this once you zoom in to 100%, and it’s rarely noticeable to the naked eye.
However, the 500 rule was developed by astrophotographers using film cameras who often wanted to produce large format prints and reproductions of their art. While small pixel movement on a screen may seem negligible, you’ll find that the traces become more noticeable (and frustrating) with a larger canvas or image.
Thus the 500 rule was born to help photographers work with the rotation of the earth rather than being thwarted by it. We thank these trailblazing photographers – and if you have any idea how to use the 500 rule on your photos, we’re pretty sure you will, too.
The 500 rule explained
So we come to the 500 rule. For starters, it’s important to mention that this is less a rule than a guideline. So don’t be afraid to make changes to suit your particular equipment and facility.
As a useful starting point, however, you should find that this will make a huge difference to the quality of your star photography, as in theory your stars should stay sharp to the edges of the field.
This is because this rule allows you to find out your maximum exposure time before stars blur or star trails appear. If you set the shutter speed slower than 500 rules, blurring or star trails will appear. (This is also especially useful when trying to get star trails – you know your minimum exposure level.)
The bad news for those of you who don’t like math is that it involves some calculations. The good news is that it isn’t particularly complicated. Depending on how you like equations, we’ve done our best to break this down.
The 500 rule
The formula is as follows:
500 divided by the Product of your harvest factor (the ratio between your sensor and a full screen) x Focal length (in millimeters) = the Ideal shutter speed.
Put more simply: Shutter speed = 500 / (crop factor x focal length)
If you broke out in a cold sweat calculating your numbers, don’t fret; We have a few examples for you.
For example, if you’re using a Canon EOS 60Da with a f / 1.8 lens, your equation looks like this:
500 / 1.6 (the crop factor of the Canon APS-C sensor) x 50 (Focal length of my lens) = 6.25 seconds
So if you are trying to avoid star trails with this rig, be sure to limit your exposure to around 6 seconds at a time to avoid star trails.
What is my harvest factor?
Your crop factor will depend on your camera, but we have a list of the most common crop factors below:
- Full frame camera = 1
- Nikon APS-C cameras = 1.5
- Canon APS-C cameras = 1.6
- Micro 4/3 cameras = 2
- Compact cameras with 1 “sensors = 2.7+
You may need to play around with the settings a little, but these numbers will help you figure out the best shutter speed for your photography.
But what about the 600 rule?
We don’t want to be glib, but the 600 rule involves exactly the same equation as the 500 rule, just with a different number.
Essentially, this means that you don’t change anything else on your rig, but you get a slightly longer exposure time.
So in the case of your equations, it will look like this Shutter speed = 600 / (crop factor x focal length)
If we look back at the example of a Canon EOS 60Da camera and the 50mm f / 1.8 camera lens, the equation looks like this:
600 / 1.6 (the crop factor of the APSC-C sensor) x 50 (Focal length of my lens) = 7.5 seconds
If I were to apply the 600 rule, I would consider a shutter speed of under 7.5 seconds for the best effect. But like the 500 rule, this is a general rule of thumb and may need further tweaking depending on your equipment. When using a Nikon or Canon with their crop sensors, you need to factor in the value of the crop sensor to come up with a useful equation.
While there is more to astrophotography than just using the 500/600 rule, it is a great entry point for your first successful photos of the stars. It limits the amount of equipment you have to invest in to capture some phenomenal pictures.
Understanding the process will enable you to deepen your knowledge of your specific equipment and enjoy the joy of capturing these heavenly moving targets.
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