Estimate maximum shutter time for sharp stars based on focal length and crop factor. This is a quick rule of thumb to prevent star trailing, not a precision tracker tool.
Formula: Rule number ÷ (focal length × crop factor). Use as a starting point; test your gear for optimal results. High-resolution sensors may show trailing sooner.
Setup: 20mm lens on APS-C camera (1.5x crop factor)
Effective focal length: 20mm × 1.5 = 30mm
500 rule: 500 ÷ 30 = 16.7s maximum
Recommendation: Use 15s or shorter for sharp point stars
Alternative: Use star tracker for longer exposures
Apparent motion of stars in long exposures due to Earth's rotation, appearing as streaks instead of points.
Smaller sensors effectively magnify the image, making star movement more apparent at the pixel level.
Yes, but expect some star trailing. For longer exposures without trailing, use a star tracker.
Start with 500, but try 400 or 300 if you notice trailing with your specific camera and lens combination.
This rule is for stars. The Moon moves faster and needs much shorter exposures to avoid blur.
These rules apply to individual exposures. Stack multiple shorter exposures for better signal-to-noise ratio.
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