Sharpening and desaturation

[sdwilsonsct]

Hi,
I notice that sharpening in Aperture 3 can desaturate.
The attached images differ only in that I have moved the (old) sharpening slider to 1.3 in the second, and get desaturation in the foreground where sharpening is applied. This is an exaggeration, but the problem shows up even at the usual low levels.
The problem is less with less sharpening , or with using a  small radius (e.g. 0.33) in the new sharpen slider.
Suggestions? Thanks!!
Scott

[Slobodan Blagojevic]

My speculation: sharpening often relies on edge halos. White halos dilute the color around them, thus the appearance of desaturation.

[sdwilsonsct]

Thanks, Slobodan.
This image:
Right half sharpened.
Bottom half: chromatic aberration turned down.

I don't see the suggested halos.
Turning down CA restores some colour.
Scott

[Slobodan Blagojevic]

Scott, I stick to my theory.

Here is a little example of what I am talking about: I created a file in Photoshop (Stubble, see below) and drew several lines, similar to your scene, with an 8 pixel yellow brush. I then sharpened it in Photoshop, using Unsharpen mask, with 500/3/0 parametars - extreme, but proves the point. See what happens with the color: disappears completely.

[sdwilsonsct]

Many thanks for this elegant demonstration, Slobodan.
Scott

[Slobodan Blagojevic]

Actually, I played with it a bit more and I think that showing several steps is even more helpful. Everything else stays the same (except for the color of the stubble), but I am showing below effects of changes in unsharp mask radius. You can clearly see how the white halo erodes the color gradually, until it totally disappears.

[Fine_Art]

So USM cranks the luminance of items close to white [255] to white? A richardson-Lucy shouldn't do that.