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?
Answer
It is taking a "black" exposure and then using it to subtract hot pixels from your long exposure image. Taking of this dark frame takes as long as was your original exposure time. So your camera is not actually processing anything there, it is just taking another image after your original image, only taking it with a closed shutter as to get an image of a warmed up sensor. Processing of these two exposures takes place after the dark frame is captured.
Setting in your camera menu is "Long Exposure Noise Reduction" or something like that. It is not "High ISO noise reduction".
Check: Why storing a long exposure photo takes almost as long as... and Does "long exposure noise reduction" option make any difference when shooting RAW? and the rest of questions mentioning LENR.
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