Published: June 20, 2024 | Last Updated: June 21, 2024
Astrophotography is capturing images of astronomical objects and celestial events.
One important aspect of astrophotography is determining the correct exposure time to avoid star trails when using a fixed tripod. A commonly used formula for this is the 500 Rule. This rule helps calculate the maximum exposure time to keep stars from trailing due to Earth’s rotation.
The 500 Rule
![\[ \text{Exposure Time (seconds)} = \frac{500}{\text{Focal Length (mm)} \times \text{Crop Factor}} \]](https://filmdaft.com/wp-content/ql-cache/quicklatex.com-1b238bd87ee2a5d7097eb6c55d580a80_l3.png "Rendered by QuickLaTeX.com")
Explanation:
500: This is a constant used in the rule. Why is the number 500, you ask? It’s arbitrary, based on experimentation, and should be used as a rule of thumb.
I recommend experimenting with a constant of 200, 300, 400 (which allows you to use shorter shutter speeds) or even 600.
Focal Length (mm): The focal length of your camera lens.
Crop Factor: This accounts for your camera’s sensor size. For full-frame sensors, the crop factor is 1. For APS-C sensors, the crop factor is typically around 1.5 or 1.6. For Micro-Four Thirds sensors, the crop factor is 2.0.
Example Calculations
If you are using a 24mm lens on a full-frame camera:
![\[ \text{Exposure Time} = \frac{500}{24 \times 1} \approx 20.8 \text{ seconds} \]](https://filmdaft.com/wp-content/ql-cache/quicklatex.com-d3b0cd9e1e48cea2634c4857ec1943aa_l3.png "Rendered by QuickLaTeX.com")
So, the maximum exposure time is 20.8 seconds before the stars start trailing.
If you are using the same lens on an APS-C camera with a crop factor of 1.5:
![\[ \text{Exposure Time} = \frac{500}{24 \times 1.5} \approx 13.9 \text{ seconds} \]](https://filmdaft.com/wp-content/ql-cache/quicklatex.com-3e1a5d732aa24626b918764812f1ffb6_l3.png "Rendered by QuickLaTeX.com")
So, the maximum exposure time is 13.9 seconds before the stars start trailing.
If you are using a 12mm lens on an MFT camera with a crop factor of 2.0:
![\[ \text{Exposure Time} = \frac{500}{12 \times 2.0} \approx 20.8 \text{ seconds} \]](https://filmdaft.com/wp-content/ql-cache/quicklatex.com-e6fd06c7168711252ee9cd2cd5feed77_l3.png "Rendered by QuickLaTeX.com")
So, like the full-frame example, the maximum exposure time is 20.8 seconds before the stars start trailing because I used half the focal length on the MFT sensor with a 2x crop factor.
Other Things to Consider
Use a wide aperture (low f-number) to allow more light to reach the sensor.
Also, increase the ISO to increase the sensor’s sensitivity to light, but be cautious of noise.
Use software to reduce noise and enhance details.
When you’re getting really hooked on astrophotography and celestial time-lapses, consider using an equatorial mount or a star tracker for longer exposures without star trails, which compensates for Earth’s rotation.
Also, try experimenting with the direction you’re shooting. Star trails (or lack thereof) change whether you’re pointing your camera North, South, East, or West.
Summing Up
The 500 Rule is a useful guideline for avoiding star trails in astrophotography.
Now, of course, star trailing can look cool. But when you want to capture the Milky Way without any blur, you need to know the 500 rule.
I recommend always taking a few test shots and refining your settings accordingly. You don’t want to have your camera taking photos for a few hours only to find out that you had the wrong settings.
