Astro NR in CNX versus Long Exposure NR in-camera
While researching the topic I ran across this old thread in the D700 forum. For reference sake it explains rather nicely what Long Exposure Noise Reduction (LENR) does....
Time marches on and now we have Astro NR in Capture NX2.
When using LENR in-camera, the RAW pixels are modified. This is one of the few (if not the only) in-camera settings that cannot be changed in CNX. For example, using High ISO NR in-camera you will find the NR applied automatically in CNX's Develop section under Camera Settings/Color Noise Reduction, and can change whatever High ISO NR was applied in-camera later if desired. However, when using LENR in-camera you will find that it does NOT cause CNX's Astro NR checkbox to become automatically checked. IOW There is no connection whatsoever between LENR in-camera and Astro NR in CNX, unlike High ISO NR.
The best description I can find from Nikon for Astro NR is.... "This option reduces the randomly spaced bright pixels that occur in images of stars captured with long exposures".
If one is to presume that Nikon's reference to "randomly spaced bright pixels" is another way of saying it does the same thing as LENR, then how can Astro NR possibly do that without the 2nd (dark) exposure from which it finds the amp noise or hot pixels to subtract from the original exposure? I don't believe in free lunches, so it's hard to conclude that Astro NR could eliminate the need to use LENR in-camera. It would be nice indeed, as it would cut exposure times in half as well as battery drain.
Does anybody know exactly what the difference is between LENR in-camera and Astro NR in CNX?
#1. "RE: Astro NR in CNX versus Long Exposure NR in-camera" | In response to Reply # 0
MotoMannequin Nikonian since 11th Jan 2006Mon 14-May-12 11:25 PM
I imagine Nikon/NIK's AstroNR is a proprietary algorithm, and other than assuming it's some sort of luminosity noise reduction specifically designed to not wipe out faint stars, which unfortunately look a lot like luminosity noise, I think there's little we can know outside of trying it to see how well it works.
LENR captures a dark frame and "subracts" it from the original frame, so it only cancels noise that's consistent between frames i.e. not random. AstroNR claims to eliminate random noise, so I don't see these techniques as being in any way related or one being a substitute for the other.
In other words, assuming AstroNR works as advertised, then astro shots could benefit from using both methods together.
It's worth noting, if you want an LENR-like effect that is not baked into the NEF, you can simply take the dark frame yourself manually by executing a same-length exopsure with the lens cap on, then subtract it in post processing. This trick is common when shooting star trails, and stacking several long exposures to simulate a multi-hour exposure. Here LENR would not work because the time to take each dark frame would remove parts of the trail, leaving it looking like a morse code message in the sky. Star trail shooters therefore take a single dark frame at the end of the entire sequence, and subtract it from the final stacked image. Find some star trail stacking tutorials (there are some really good ones out there) to get some more detailed info on how to accomplish LENR in post-processing.
Larry - a Bay Area Nikonian
My Nikonians gallery
#2. "RE: Astro NR in CNX versus Long Exposure NR in-camera" | In response to Reply # 1
FrankSRGB Nikonian since 09th Aug 2009Wed 16-May-12 12:42 PM
>I imagine Nikon/NIK's AstroNR is a proprietary algorithm, and other than assuming it's some sort of luminosity noise reduction specifically designed to not wipe out faint stars, which unfortunately look a lot like luminosity noise, I think there's little we can know outside of trying it to see how well it works.
Thanks for confirming my suspicions. It sounds like you couldn't find any more documentation than I could.
As you've suggested, I've conducted some tests in CNX. The test image I used was from a D700, 3200 ISO, 20 second exposure with LENR and High ISO NR active.
Unfortunately Astro NR does wipe out faint stars and will dim some of those that are less faint. It's subtle but definitely there. It will do that even when the High ISO NR Intensity & Sharpness values are set to zero, and without any perceivable difference in noise surrounding those stars or elsewhere in the sky. Of course, using the Intensity/Sharpness settings will wipe out more stars, but unlike Astro does reduce the surrounding noise as well as soften the entire image.
Frankly, I don't see any use for Astro as it did nothing to improve this image, it only removed stars. Until Nikon sees fit to supply details, I have to assume Astro is just a low quality hack/substitute for LENR, meant for those times when LENR should have been used but wasn't.
I am aware of the other techniques you've mentioned, but do appreciate you taking the time to write about them.