Nikon D300 User Manual page 203

My Recommendation: I personally like the benefits of Long
exp. NR. I shoot a lot of waterfall and stream shots where I
need exposures longer than 8 seconds to really blur the water.
Also, I like to take midnight shots of the sky, and even shots
of city scenes at night. Even though it may slow down the
frame rate slightly and give me one less image in the
in-camera memory buffer, I still use it most of the time.
If I were a sports or action shooter using Continuous release
mode, I may leave Long exp. NR set to Off. It's unlikely I
would be using exposures longer than 8 seconds, and I would
want maximum frames per second as well as the ability to
cram as many images into the camera buffer as possible. I
wouldn't want my camera to slow down while writing images
to the memory card.
Your style of shooting will govern whether this function is
useful to you. Ask yourself one simple question, "Do I often
shoot exposures longer than 8 seconds?" If so, you may want
Long exp. NR set to On. Compare how the images look with
and without it. I think you'll like Long exp. NR.
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