Why does a digital camera take just as long to process a long-exposure shot?
Asked 10/12/2013
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On many digital cameras, a long exposure seems to take the same amount of time again after the shutter closes before the image is saved. For example, a 15-second exposure may require about 15 more seconds, and a 30-second exposure may require another 30 seconds afterward.
What is the camera actually doing during that extra time? Is it capturing multiple frames and combining them, or something else? How does this work for very long BULB exposures where the camera clearly cannot be buffering lots of full images in memory?
I’m looking for the technical explanation of what causes this one-to-one delay after long exposures.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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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.
Originally by user17441. Source · Licensed CC BY-SA 4.0
user17441
12y ago
0
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Usually that extra time is not the camera combining many frames. It’s doing long-exposure noise reduction by taking a second exposure of equal length with the shutter closed, often called a dark frame.
That dark frame records sensor noise that becomes prominent during long exposures, especially hot pixels and thermal noise from the warmed sensor. The camera then subtracts that dark frame from the real exposure before saving the image. Because the dark frame must be the same duration as the original shot, a 15-second exposure needs about 15 more seconds, a 30-second exposure needs about 30 more, and so on.
So for an 8-minute BULB exposure, the camera may spend roughly another 8 minutes capturing the dark frame, then process the subtraction afterward.
This behavior is typically controlled by a menu setting such as Long Exposure Noise Reduction. It is different from High ISO Noise Reduction. If long-exposure NR is disabled, the camera generally won’t need that matching post-shot wait.
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