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This is similar to this question, but I think slightly different. When post-processing a photo, especially one taken at high ISO, how much noise reduction should be applied?

I know this question can depend some on artistic choice, and it also can depend some on the output format, so primarily I'm interested in the "technically correct" answer (so I can deviate from it deliberately instead of accidentally), as well as the most "general-purpose" answer (if I need to apply it for a specific medium, I can always go back to the original and tweak things for that format).

Edit: Based on the answers below, I guess "techinically correct" isn't the right term I'm looking for. Basically what I'm trying to get at is "How much is too much? How much is too little?" I recognize that each photo is going to be different, but right now I feel like I'm just guessing -- are there any guidelines or best practices, or default settings that people use as a starting point?

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There is no "technically correct" answer either, for a number of reasons:

  • Noise in images varies based on the actual exposure, and unless every image you take is evenly lit across the frame, the right amount of NR will vary.
  • Different sensors have different noise characteristics. This year's models are so much better than those from five years ago that any fixed answer would have to change anyway. This isn't just in the quantity of noise, but in the amount and character of the noise.
  • Noise reduction comes at the expense of detail. The "technically correct" answer varies depending on where you want to make that tradeoff for each image.
  • Noise reduction algorithms will affect different details differently, and human perception of the change may be better or worse. Generally, I can tolerate a lot less noise reduction smoothing in human faces than I can in other subjects, for example.
  • Different output media (for prints and even for digital viewing) tolerate different levels of noise.

If there were a technically correct answer, all software would just do that. But there isn't.

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  • \$\begingroup\$ Ok, this is a fair point. I was attempting to make the question less subjective, but this way of phrasing made it seem like I was expecting a "one-size-fits-all" solution. I do recognize that this is a subjective issue that may depend a lot on the photo, but I'm trying to figure out what is "generally accepted" as looking good. Or should the take-home message be that there is no generally accepted "good" amount of noise? \$\endgroup\$
    – David
    Commented Jul 3, 2012 at 15:51
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    \$\begingroup\$ Matt is right. There is no generally good level of noise. Some people like less noise and don't mind the softness added by noise-reduction and others prefer the reverse. Even more important is that noise shows up very differently depending on the print-size and medium used. Textured papers (canvas style) for example tend to hide noise quite well, smooth matte papers not so much. \$\endgroup\$
    – Itai
    Commented Jul 3, 2012 at 15:56
  • \$\begingroup\$ There's generally a level of noise below which you don't really want to fall as well. If you've ever done the "perfect" 3D render or done a background swap (to anything other than pure black or white), you've probably noticed that the eye won't accept it as real until you add just a hint of noise/grain in the luma channel (chroma noise is less desirable). It's not just about quantization artifacts, but that is a part of it. That tiny amount of dither might bother you at the pixel-peeping level, but not nearly as much as its absence will at the "big picture" level. \$\endgroup\$
    – user2719
    Commented Jul 3, 2012 at 19:15
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To provide another way of looking at the answer:

The technically-correct baseline value is none — and then adjust to taste.

Noise reduction is inherently destructive of detail, so start without any and increase as warranted by the individual characteristics of each photograph and to match your taste. You may find that you're always using some small amount; in that case, you may want to adjust your baseline up to that minimum. But low should be the default — as the saying goes, you can't add less salt.

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I used to just crank the luminance slider all the way up, but this tends to create plastic-looking photos, so now what I do is move the luminance slider until I can't see any grain, and then back it down by about 10 so that there's just a hint of grain visible.

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Every camera would respond different at 800 ISO for ex. so you can't establish a mark from where to start, in adition if you use always the same software and camera, you can establish a mark, from where you know, if you take some pictures in 800 ISO, you apply noise reduction to "delete" noise by 4 stops, and so for others ISO stops.

There is no technically correct way, is your way to do it.

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