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Hi Shane
That sounds like you have “Noise Reduction” turned on?
When a modern DSLR takes an exposure of longer than a few seconds, it usually takes an equally long exposure “with the lens cap on” as it were.
This 2nd (automatic exposure) is an image of the “thermal noise” on the sensor which it then subtracts from your previous exposures, giving you a hopefully cleaner photo.
On some cameras, there is a Menu function to turn off this automatic “noise reduction” or “dark” frame.
Cheers
Dennis
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