I've noted obvious noise in the sky of images scanned from under- exposed slides, characterized by very blotchy, mottled colors, & distinct "dots", that if not treated with a noise reduction procedure, are unacceptable.
With properly exposed slides, on the other hand, the sky will generally print clean, without a hint of noise; though if you observe them at the pixel level they are by no means the smooth transition of colors as viewed in the print, but instead smooth, yet myriad variations of the colors in the sky, including blues, whites, pinks, etc.; The overall perception is not unlike a noisey image that's been noise filtered to a reasonable point far short of plactisizing.
The question is, since any amount of noise filtering diminishes detail, and sharpening in the sky (blue channel) typically adds noise, is it wise to apply a noise filter to such an image, in the first place?
I wondered if there were any examples of sky "shots" depicting where filtering would be prudent, vs "left as is"?
Leigh