I've never seen sharpness affected by the long exposure NR, but it's purpose is to eliminate the noise you'll see in extremely long exposures. I wouldn't leave it on in general due to the delays it causes (it doubles the time for an exposure), but it's extremely worthwhile to have it on for long exposures.
No effect on sharpness whatsoever. It merely subtracts out black sensor noise. The real disadvantage is that it doubles the exposure black out time - thus a 20 second exposure becomes a 40 sec exposure.
That doesn't look like it's affected by amp noise, so probably no effect at all. What Long Exp NR does is to eliminate the pinkish blobs that sometimes seem to intrude on the edges of frames with long exposures. It's caused by thermal buildup on electronic components near the sensor, so the various cameras have different liabilities in this regard. (Presumably also the various sensors affect this too.)
The way it works is to take your exposure, then leave the shutter closed and captures another one that should be all blank. If amp noise is affecting the sensor, the camera can safely conclude that anything in the blank exposure should not be, and it subtracts that from your exposure before it saves it to flash memory.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!
I know this is an old thread but am hoping someone here has explored the differences between Long Exposure NR and Capture NX2's new Astro NR. If so, could you please look at this thread in the Digital Processing forum and share your knowledge?