Sharpening in LAB Mode

[John Chardine]

Could you give an idea of how much surface blurring is appropriate here. Where is a good starting point?

Actually, if you go too far, you will still get halos and so should adjust the Radius as necessary. In "the" LAB book by Dan Margulis, it also suggests blurring the "a" and "b" channels with Surface Blur to reduce noise (especially "b"). That's what I've been doing, blur "a" and "b" before sharpening the lightness channel. LAB sharpening is all I use now.

[Desmond Chan]

Could you give an idea of how much surface blurring is appropriate here. Where is a good starting point?

Looking back, I think I've made a mistake. My apologies. I think I made it look like one should blur the a and b channel every time before the sharpening. The idea to blur the a and b channel is to hide the noise. In LAB, the noise usually shows up in b ( in the blue channel in the case of RGB and b control the blue-yellow axis). So if your photo does not have a lot of noise, I don't think you need to blur either the a or b channel. Or you can simply use other methods to reduce the noise first. As for how much to blur if you do need to, I myself cannot give you any guidance on it. Dan Margulis in his book uses surface blur to blur the a and b. So I suppose you have to know something about surface blur before you have some idea on the settings.

If you want to know more about LAB, I suggest you to get a copy of Dan Margulis' book. Mind you, his books don't seem to be easy to read. They have a lot of details though. They're advanced materials.