The “500 Rule” is a guideline used in astrophotography to determine the maximum exposure time to avoid star trailing. It helps ensure that stars appear as pinpoint dots rather than elongated streaks in long-exposure images. The rule is as follows:
Exposure Time (in seconds) = 500 / (Focal Length * Crop Factor)
Here’s a breakdown of the components:
- Exposure Time: The recommended maximum exposure time in seconds for a single shot.
- Focal Length: The focal length of your lens or telescope in millimeters.
- Crop Factor: The crop factor of your camera, accounts for the sensor size. For full-frame cameras, the crop factor is 1. For crop-sensor cameras, such as APS-C or Micro Four Thirds, the crop factor is typically around 1.5 to 2. This factor is used to calculate the effective focal length.
By applying the 500 Rule, you can estimate the maximum exposure time before stars begin to trail. However, it’s worth noting that this is a rough guideline and not an exact science. Different factors, such as the level of magnification, the declination of the object being photographed, and personal preferences, may require adjustments to the rule.
Additionally, with the advancement of camera technology and image stabilization techniques, astrophotographers often use more precise methods like the NPF Rule (Nightscape Photography Formula) or employ auto-guiding techniques for longer exposures without noticeable star trailing.
