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I'm looking for a technical answer, ideally from an engineer that actually works on the cameras or someone who's reverse engineered the firmware. I'm a graphics programmer so don't hold back the tech.
On pretty much every digital camera I've owned, whatever speed I set the exposure too to the camera takes that much time to process the image after it's stopped capturing data. In other words if I take 15" exposure the camera will take 30" total, 15 for capturing the image and another 15 for processing. If I take a 30 second exposure it will take 30 seconds for capturing the image and 30 more for processing.
So, what's really happening? I can imagine the camera is actually capturing multiple images and merging them. But if that's the case how many images is it capturing? At what frame rate? If I use BULB and do a 8 minute exposure there's no way the camera has enough memory to capture that many frames. What happens then?
To state the question again, what is the camera doing when "processing" a long exposure photo?