Do you want to create beautiful photos of the night sky? The 500 Rule is a great astrophotography technique that’ll help you render sharp stars, consistently. Best of all, it’s ultra easy to use.
So if you’re new to the 500 Rule, you’re in luck; this article will guide you through the main principles, explain how to use it, and highlight the importance of applying it in your nighttime photography.
Let’s get started.
What is the 500 Rule?
The 500 Rule is a popular guideline for photographing stars. Its main aim is to capture stars as razor-sharp pinpoints rather than star trails – by helping you calculate the longest shutter speed you can use without noticeable star streaking.
You see, photographing sharp stars – as opposed to slight star trails – requires a reasonably fast shutter speed. If your shutter speed gets too low, your stars will start to streak. And while this can look nice when done carefully, unintentional star trails can become messy.
Why is the 500 Rule important?
Truthfully, the 500 Rule, despite its name, is really more of a guideline. But it’s still plenty important, because it helps you capture striking images of the night sky without streaky stars.
You see, when you’re photographing at night, your camera is often starved for light – and so the longer you can make the exposure, the better. But go too long and you get star streaks, which is where the 500 Rule comes in; it provides you with a cutoff point that you can’t (or shouldn’t) go past when doing astrophotography.
The 500 Rule: the basic calculation
So, how does the 500 Rule work?
In principle, the 500 Rule is easy to calculate. Simply take 500 and divide it by the focal length of your lens. The result is equal to your maximum shutter speed.
For example, when using a lens with a focal length of 24mm, you would divide 500 by 24, which gives you 21 – or a shutter speed of 21 seconds.
And when using a 50mm focal length, you would divide 500 by 50, for a maximum shutter speed of 10 seconds.
It is important to remember, however, that the 500 Rule is only an approximation of the best exposure time to use. It won’t always guarantee that your images will be free of star trails (and later on in this article, I discuss some situations where the 500 Rule can fail).
When to use the 500 Rule
The most popular time to use the 500 Rule formula is when shooting night scenes of static stars or the Milky Way. When photographing the night sky, the conditions are very dark, and you will need to adjust your camera settings to compensate for the low light. Usually, if you want to capture a bright enough image of a night scene, you will need to increase the exposure time accordingly (and you should keep the ISO low to minimize noise).
This is exactly the time to apply the 500 Rule. By using the maximum shutter speed, you’ll get sharp stars – and you’ll also get a well-exposed image with the minimum amount of noise.
Generally, you’ll want to use a fairly wide-angle lens when photographing the night sky because it’ll offer a broader field of view. I recommend a shorter focal length lens such as a 14mm, 16mm, or 24mm to capture more stars in the scene.
For your convenience, I’ve calculated out the maximum exposure times for each of these focal lengths:
- 14mm | 500/14 = 35 seconds
- 16mm | 500/16 = 31 seconds
- 24mm | 500/24 = 20 seconds
Dealing with crop-sensor and Four Thirds cameras
The 500 Rule is a great concept, one that you can use with any camera. However, the basic 500 Rule calculation mentioned above applies solely to full-frame cameras, and you’ll need a different formula for crop-sensor and Four Thirds cameras.
If you use a crop-sensor or Four Thirds camera, the maximum shutter speed can be calculated as follows:
500 divided by the crop factor multiplied by the focal length.
So first multiply the focal length by the crop factor (which gives you the effective focal length). Then divide 500 by the result.
For a Canon APS-C camera and a 24mm lens, this would be 500 divided by 1.6 x 24 = 13 seconds.
For Nikon APS-C camera and a 24mm lens, this is 500 divided by 1.5 x 24 = 14 seconds.
The Four Thirds crop factor is 2, so with a 24mm lens, you get 500 divided by 2 x 24 = 10 seconds.
Should you always use the 500 Rule for astrophotography?
You shouldn’t always use the 500 Rule. Sometimes, the suggested maximum shutter speed can still result in star trails in your images. For example, according to the 500 Rule, a 12mm lens should let you photograph down to around 40 seconds or so without star trails, yet an exposure time of over 30 seconds can show star movement. The same is true of an ultra-wide 14mm lens.
So if you want to record sharp stars, you may want to avoid using the 500 Rule with such lenses.
Alternatively, you may be someone who prefers to shoot star trails. If that’s the case, there is no need for you to use the 500 Rule. Instead, use an ultra-long shutter speed to record the curve of the stars as they move. You’ll generally need to apply a shutter speed of 30 seconds up to an hour or more.
(The longer the exposure, the larger the star trail!)
Also, the 500 Rule does not take into consideration other factors such as light pollution, haze, and the angle of the stars. Therefore, even after making your 500 Rule calculation, it’s best to experiment with the shutter speed for optimal results.
The 500 Rule: final words
The 500 Rule is a great technique, and you can use it to achieve beautifully sharp shots of stars. It’s a clever method of finding the maximum exposure based on your camera type and lens focal length, and it really is a brilliant way of improving your photos of the Milky Way and the night sky.
If you own an APS-C or Four Thirds camera, be sure to adjust the calculation for the relevant crop factor. Above all, enjoy shooting the night sky!
Now over to you:
What do you think of the 500 Rule? Do you plan to use it in your astrophotography? Do you have any tips? Share your thoughts in the comments below!
