Help with B&W Conversion Issue

[JuSlaughter]

Attached is a crop of an image I took tonight. When I convert to b&w, using either desaturation or channel mixer, I get this pixilation in the sky. So I have 2 questions.

1. Why?
2. Best way of getting rid of it?



Cheers

Ju

[poloman]

I would try cutting out the buildings and doing noise reduction on the sky. You might get rid of it with gaussian blur. I couldn't access the exif data so I don't know what your settings where. Did you lighten it significantly in post processing? The image has a lot of noise in other areas as well. Was this a high ISO shot?

[Radtech1]

That's not pixilation, it's posterization.

It happens because the sky is dark mostly on the blue channel. At that brightness (or darkness), there are only so many low luminosity densities of blue available to an 8 bit display.

What I do is I keep a color layer available after I convert to mono. Then I use the blue sky in the color layer to accurately select the sky.

Move that selection to the mono layer, copy and paste in that selection - which should be just the black and white sky.

Then Gaussian Blur that layer. To eliminate the blur from bleeding, then just re-select, invert the selection, then cut. That will remove whatever blur extended beyond the original selection.

Rad