The 500 second rule in astrophotography is a practical guideline that helps photographers determine the maximum exposure time for untracked night sky photos before stars begin to appear as streaks or trails due to the Earth's rotation. It's an essential tool for capturing sharp, pinpoint stars without the need for an equatorial mount.
Understanding the 500 Second Rule
When photographing the night sky without a star tracker, the Earth's rotation causes the stars to move across the frame. If your exposure is too long, these stars will blur into short lines rather than sharp points of light. The 500 rule provides a simple calculation to estimate this maximum exposure time.
How it works:
To apply the rule, you take the number 500 and divide it by the focal length of your lens. The result is the approximate maximum exposure time in seconds.
- Purpose: To prevent star trailing in untracked astrophotography.
- Application: Used with a camera on a stationary tripod.
- Calculation:
Maximum Exposure Time (seconds) = 500 / Lens Focal Length (mm)
Example Calculation
Let's illustrate with an example:
| Lens Focal Length (mm) | Calculation | Maximum Exposure Time (seconds) |
|---|---|---|
| 20mm | 500 / 20 | 25 |
| 14mm | 500 / 14 | 35.7 (approx. 35 seconds) |
| 35mm | 500 / 35 | 14.2 (approx. 14 seconds) |
| 50mm | 500 / 50 | 10 |
As the example shows, if you are using a 20-mm wide-angle lens, your maximum exposure time before noticeable star trailing will be approximately 25 seconds. Using a wider lens allows for longer exposure times, which is why wide-angle lenses are popular for capturing vast stretches of the night sky.
Why the Rule is Important
- Sharp Stars: Ensures stars appear as crisp, individual points of light.
- Untracked Photography: Crucial for photographers working without a star tracker, which automatically compensates for Earth's rotation.
- Starting Point: Provides a reliable baseline for your exposure settings, which can then be fine-tuned based on your specific results and preferences.
Factors Affecting Star Trailing Beyond the 500 Rule
While the 500 rule is widely used, it's a simplification and serves as a good starting point. Several other factors can influence when star trails become noticeable:
- Sensor Size (Crop Factor): The rule is traditionally based on full-frame cameras. For cameras with APS-C or Micro Four Thirds sensors, you should typically divide by your lens's effective focal length (actual focal length multiplied by the crop factor). For instance, a 20mm lens on an APS-C camera with a 1.5x crop factor effectively behaves like a 30mm lens, reducing the allowable exposure time. Some photographers opt for the "300 rule" or "400 rule" for crop sensors.
- Pixel Pitch: Cameras with smaller pixels will show trailing sooner than those with larger pixels at the same magnification, as smaller pixels capture finer detail.
- Desired Level of Sharpness: What one photographer considers a "pinpoint" star, another might see as slightly trailed. Your personal tolerance for trailing can lead you to adjust the exposure time slightly.
- Direction of Photography: Stars closer to the celestial pole (e.g., Polaris in the Northern Hemisphere) appear to move slower and can tolerate longer exposures than stars closer to the celestial equator.
Practical Tips for Astrophotography
- Use Wide-Angle Lenses: Lenses with shorter focal lengths (e.g., 14mm, 20mm) are highly recommended as they allow for longer exposure times, capturing more light and detail from the night sky.
- Set ISO and Aperture: Pair your calculated exposure time with a wide aperture (e.g., f/2.8, f/4) to gather maximum light, and adjust your ISO sensitivity (e.g., ISO 1600, 3200, 6400) to achieve proper brightness without excessive noise.
- Review Your Images: Always review your photos at 100% zoom on your camera's LCD screen or computer to check for star trails and adjust your exposure time if necessary.
- Experiment: The 500 rule is a guideline. Don't hesitate to take test shots slightly above or below the recommended time to find what works best for your specific gear and desired outcome.
By understanding and applying the 500 second rule, astrophotographers can effectively capture stunning, sharp images of the night sky even without specialized tracking equipment.
