Why does a 15–30 second exposure take just as long to finish saving?
Asked 8/12/2012
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When I shoot long exposures at night (around 15–30 seconds), the camera seems busy for about the same amount of time after the shot before the JPEG is saved. The files are only about 1–3MB, so it doesn’t seem like card write speed is the issue. Why does this happen?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
50
This is a featured called Long Exposure Noise Reduction. To cancel out noise the camera will close the shutter and take an equally long exposure again, this time capturing a black image with only the electrical noise on it. This information is then used to reduce the noise on the original exposure.
In the camera settings you can disable the Noise Reduction for long exposures. Check the manual or google to find out how to enable and disable it.
Originally by user11091. Source · Licensed CC BY-SA 4.0
user11091
14y ago
0
Generated from our catalog & community — verify before relying on it.
This is usually caused by Long Exposure Noise Reduction being enabled, not by slow card writing.
With this feature on, the camera makes your normal exposure, then immediately takes a second exposure of equal length with the shutter closed. That second “dark frame” records sensor noise and hot pixels, and the camera subtracts it from the original image. So a 30-second exposure can require roughly another 30 seconds of processing.
This is why the delay matches the exposure time even when the JPEG file is small.
Check your camera menu or manual for Long Exposure Noise Reduction (sometimes called dark-frame subtraction). You can usually disable it if you want faster shooting, but you may see more noise in long exposures as a result.
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