Exposure Rule.
Use this rule to minimise star trailing in your night sky photographs
Exposure in seconds = R divided by the focal length of the lens
For example the exposure time for an APS-C camera using a 24mm wide angle lens is-
330 / 24 = 13.75
Round this value down to the nearest exposure value in your cameras settings.
In this case 13 seconds.
Full Frame Sensor Cameras
R = 500
APS-C Sensor Cameras
R = 330
Micro Four Thirds Sensor Cameras
R = 250
One Inch Sensor Cameras
R = 185
Compact and Bridge Cameras
R = 90
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