Why Are My Star Photos Trailing Instead of Sharp?

Likely causes

• Shutter speed too slow for the focal length used. the longer the exposure and the longer the lens, the more visible the Earth's rotation becomes as star trailing.
• Using a rule of thumb that doesn't fit the actual sensor. an older or simpler rule like the basic 500 Rule can be too generous for a modern high-resolution sensor viewed at full size.
• Not rechecking after a focal length change. switching to a longer lens partway through a shoot shortens the maximum safe shutter speed before trailing appears.

The fix

1. Check the maximum shutter speed for pinpoint stars at your specific focal length before shooting.
2. Use the stricter NPF Rule instead of the basic 500 Rule if shooting on a high-resolution modern sensor.
3. Recalculate the limit whenever focal length changes rather than reusing an earlier number.