sharpening

[obelisk]

What is the sharpening measured in with ctrl-U?
I shrink my files to 1800px, put sharpening 10,20,30. I could hardly see any difference between them, and couldn't say which is sharper. How should I use this?

[xnview]

Sharpen is a filter 3x3, so for large image it's difficult to see it

[obelisk]

I see. I have a big file, and then I would shrink them with xnview for:
1) web 1200x1800 + wm
2) client files 900x1500 approx
2) client files 1800x2800 approx

What is your advice for sharpening these?

[XnTriq]

Enhance Focus and Focus Restoration (Filter » Effects… » Enhance) have served me well.

• Free unsharp mask plugin?