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Does your original image still have noise if you zoom in to 1:1? What is the original image ISO and what Luminance Noise-Reduction Amount are you using to mitigate the noise?
I'd guess this is a very high ISO image and the noise in the original isn't being quashed completely and you may be oversharpening during Export.
The larger preview does actually have quite a bit of noise, too, if you look at it closely enough. Here are a couple screenshots of the upper-lefthand corner and ear area of the lower image you included, resampled up 2:1 so you can see the detail on your high-res monitor:
The main reason you can see the noise so visibly is that the OS is using a fast but inaccurate resampling algorithm to generate the previews from the much larger image, and this fast algorithm doesn't average multiple pixels to generate each pixel in the smaller version. This tends to preserve hard edges which is good for line art, but it accentuates micro texture in what should be a smooth-toned image.
If you want further analysis, then show us a screenshot at 1:1 in Lightroom of say the ear area along with the Detail section settings you're using for Sharpness and Noise-Reduction in Develop, then show us your Export panel settings that include what resizing and sharpening you're doing, there, and finally upload the Exported JPG to www.dropbox.com and include a public-share link to it, here.
Thank you ssprengel for your reply.
(screenshot zoom 1:1 of the raw file)
Here's the zoom 1:1 of the shot on Develop with the Detail section
And here's the Export settings of the JPG
The original RAW file and the exported JPG are here https://meocloud.pt/link/12ebbcc3-9730-4257-810f-92090cfa677e/Files/
Let me know what you think.
Thank you again.
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Beside the inaccurate resampling algorithm in OSX Preview app, the main issue I see is that Export (output) sharpening is being applied without the image having been resized.
In general, Export sharpening is used to counteract the blurring due to resizing smaller, but you're not resizing smaller, so instead the sharpening is merely enhancing the remaining noise in the image, reversing the effect of the noise reduction or perhaps even making the noise worse than the original.
Here is a side-by-side of your original raw with the same Detail settings applied in your screenshot along with your exported JPG with no changes. As you can see the noise in the exported JPG is much worse than the raw with NR applied. This difference in noise is because of the added Export sharpening.
The other non-optimal thing I see is that you're not using any Masking in the Develop Sharpen area, so any sharpening being applied in Develop is applying equally to the image details and the noise, which is probably not what you want with a high-ISO image.
You can hold down the Alt or Opt key while dragging the Masking slider until just edges are highlighted:
Here is the result of adding this masking, it reduces the noise everywhere except the edges highlighted above that still remain sharpened:
After using masking, perhaps you can dial back the noise reduction a little more.
The OS Preview app is still enhancing the noise due to using the fast but inaccurate resampling algorithm as described earlier, but this reply is about how the exported image, itself, also has much more noise than you perhaps intended.
Why this just started happening, I don't know. Maybe you started sharpening during Export for whatever reason, or maybe you started shooting at high ISO.
If you had set your Export sharpening to Matte Standard as shown in the screenshot because you will be printing on matte paper, then understand that the sharpening you do before printing should be done at the printer resolution not the native image resolution, so if this Export is for sending to a printer rather than viewing, it will need to be sharper than what looks good on the screen, but you should be resizing using the pixel dimensions or Inches/PPI that make sense for the final print size and making a different export for each different print size. An 8x10 print at 300PPI should be resized to 2400x3000 along with being sharpened during Export.
It is much easier to let LR do this resizing and sharpening in the Print module if you can print from LR.
If the exported image is just for screen viewing then it should be resized down to screen size and then sharpened if needed, and if this image is just an intermediary without knowing what output medium it is for, then don't use Export sharpening at all if you are exporting at the native size (i.e. without resizing).
Thank you so much for you reply.
I'm going to try it and let you know how it worked out.