The 500 rule is critical to remember for astrophotography because it helps you determine the maximum shutter speed you can use without getting star trails in your images. Star trails occur when stars appear as streaks or lines in your photos, rather than as pinpoint dots of light. This is caused by the Earth's rotation, which causes the stars to move across the sky during long exposures. 

By using the 500 rule, you can calculate the maximum shutter speed you can use before the stars start to blur. This allows you to capture sharp, detailed images of the night sky, with the stars appearing as crisp points of light rather than streaks. 

While there are other factors to consider when photographing the night sky, such as aperture, ISO, and focus, the 500 rule provides a simple and effective starting point for determining the optimal exposure settings for your camera and lens. By keeping this rule in mind, you can ensure that your astrophotography images are sharp, detailed, and free from distracting star trails. 

As we are talking astrophotography here, you can read another article I have on the topic.

What is the 500 rule? 

The 500 rule is a simple equation that helps photographers determine the optimal shutter speed to use when photographing the night sky. Essentially, the rule states that in order to capture sharp stars without blurring, you should use a shutter speed no longer than 500 divided by the focal length of your lens. 

For example, if you are using a 24mm lens, the calculation would be 500 ÷ 24 = 20.8. Therefore, you should use a shutter speed of 20 seconds or less to avoid star trails. 

Here are the steps to follow to use the 500 rule: 

1. Set your camera to manual mode. 

2. Choose your aperture and ISO settings. 

3. Determine the focal length of your lens. 

4. Divide 500 by the focal length to get the maximum shutter speed in seconds. 

5. Set your shutter speed to the calculated value or shorter. 

Table of focal lengths and shutter speeds 

Here is a table with different focal lengths and their corresponding maximum shutter speeds based on the 500 rule: 

 Remember that these shutter speeds are the maximum you should use to avoid star trails when photographing the night sky. Depending on the lighting conditions and other factors, you may need to shoot at a shorter shutter speed to achieve the desired exposure. Additionally, these values assume a full-frame camera sensor, so if you're using a camera with a crop sensor, you'll need to adjust the calculation accordingly. 

Here are a few additional tips for using the 500 rule 

• If you are using a crop-sensor camera, you will need to take the crop factor into account when calculating the focal length. For example, if you have a 1.5x crop factor and a 24mm lens, your effective focal length would be 36mm (24 x 1.5). 

• If you are using a wide-angle lens, you can often get away with using a slightly longer shutter speed than the 500 rule suggests. 

• If you are using a telephoto lens, you may need to use a shorter shutter speed than the 500 rule suggests to avoid star trails. 

• Use a sturdy tripod: In order to avoid camera shake during long exposures, it's wise to use a sturdy tripod that can support the weight of your camera and lens. 

• Consider using a remote shutter release: A remote shutter release can help you avoid touching the camera and causing it to shake during the exposure. 

• Experiment with different shutter speeds: While the 500 rule provides an ideal starting point, don't be afraid to experiment with different shutter speeds to achieve the desired effect in your photos. 

• Pay attention to the moon phase: The brightness of the moon can affect the exposure of your night sky photos. If the moon is full or nearly full, you may need to use a shorter shutter speed to avoid overexposing the image. 

• Shoot in RAW format: Shooting in RAW format can give you more flexibility when it comes to adjusting the exposure and colour balance of your images in post-processing. 

• Practice, practice, practice: As with any form of photography, practice is key to improving your skills and getting the results you want. Don't be discouraged if your first attempts at astrophotography don't turn out exactly as you hoped – keep experimenting and learning from your mistakes. 

500 rule in photography – conclusion 

In conclusion, the 500 rule is a simple and effective tool for astrophotographers who want to capture sharp, detailed images of the night sky. By using this rule to calculate the maximum shutter speed you can use without getting star trails in your images, you can achieve stunning results that capture the beauty and majesty of the cosmos. 

Of course, there are many other factors to consider when photographing the night sky, from the brightness of the moon to the clarity of the atmosphere. But by starting with the 500 rule and experimenting with different settings and techniques, you can hone your skills and create images that capture the wonder and magic of the universe. 

So the next time you venture out into the night with your camera in hand, remember the 500 rule and use it to guide your exposure settings. With a bit of patience, practice, and creativity, you can create stunning astrophotography images that will amaze and inspire those who view them.