Why does my Nikon take longer to show or save long exposures?
Asked 7/16/2013
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On my Nikon D3100, when I use a long shutter speed (for example 20 seconds), the camera seems to take longer before the image appears or finishes saving. I first thought this meant the sensor readout itself was slower. Is sensor readout time related to exposure time, or is the delay caused by something else?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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There's no fundamental link between readout and exposure time.
Long exposures do tent to be noisier however, some cameras have a long exposure noise reduction feature that captures an equally long exposure but with the shutter closed, in an attempt to capture any signal generated by heat, so that signal can be subtracted from the shutter open image.
You'd know if this was the problem as it would take a whole 20 seconds for the image to be produced!
Other than that it's possible the camera is doing some time consuming software noise reduction that doesn't involve a blackframe, or that a noisy image takes longer to write to the card as it benefits less from lossless compression of the RAW or JPEG data (the extra time ought to be marginal though).
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
0
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Sensor readout is not fundamentally tied to exposure length. A 20-second exposure does not inherently make the sensor itself read out more slowly.
What you’re likely noticing is processing or write delay after the exposure ends. Common causes are:
- long exposure noise reduction (LENR): the camera makes a second “dark frame” exposure of the same length with the shutter closed, then subtracts noise from the original image. If you shoot 20 seconds, this can add about another 20 seconds before the image is ready.
- image processing/compression: noisier images, especially at higher ISO, may compress less efficiently, so RAW/JPEG files can be larger and take a bit longer to process or write to the card.
- card write time: the camera may simply still be writing the file to the memory card.
So the delay is usually not slower readout, but post-capture processing and storage. If the wait is almost exactly as long as the exposure, check whether long exposure noise reduction is enabled.
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