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I usually took long exposure photo with ISO 100, so do I really need to turn on Long Exposure NR on? It really take time for processing.

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The long exposure Noise Reduction takes a "dark frame" after the real shot, and then uses the noise pattern from that image to reduce the noise generated by the sensor. That's why the exposure takes twice as long as this NR were disabled.

Long time exposure can increase the sensor temperature, and increased temperature also means more noise. So you should generally leave that feature on, and disable only if time is a critical issue.

The dark-frame is (depending on the camera manufacturer) also used to identify and map out any hot pixels during the exposure.

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Long exposure noise reduction is actually dark-frame subtraction done automatically in-camera.

If you are taking many shots and would like to avoid the time delay, you can take just one dark frame, with the lens cap on in the same conditions and exposure time, at the end of the shooting and then use dark-frame subtraction in Photoshop, Lightroom, GIMP and other software. Essentially, you are subtracting sensor noise from your image.

This also makes it easy to see the result with and without dark-frame subtraction.

I use and recommend it.

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