Sharpening - Part 2 - Separate control over light and dark halos

ruttrutt Cave canem!Registered Users Posts: 6,511 Major grins
edited May 2, 2005 in Finishing School

In Part 1, I explained the basics of sharpening and led you through the process of finding good values for USM. Now I'm going to teach you a new technique that will allow you to sharpen more in many cases without over sharpening. This could be described as an advanced technique, but it is the most common enhancement to basic USM there is. Virtually every prepress professional knows this trick, and soon you will, too.

Here is an before/after of an image sharpened using this technique:



Notice the how well the illusion of sharpness works even at this size.Look especially at the stonework on the bridge. Yet the branches of the trees don't look over sharpened, which is the most common bugaboo of sharpening.

The idea is to gain independent control over the light and dark halos.Usually the light halos are more easily detected. In order to prevent visible light halos, we are restrained to more conservative USM values. But once we can independently specify the values for the light and dark halos, we can make the dark halos do their job without making the light halos give away the trick.

Quick start

The basic idea is really simple. Make 2 layers, one for the dark and and one for the light halos. We can do this by setting the blending options of the layers, one to Lighten and one to Darken. Then we can control the opacity of the light and dark halos independently by changing the opacity of each layer separately. I'll walk through the process that produced the image above in some detail, but in essence here are the steps.
  1. Make a duplicate layer.
  2. Sharpen the L channel of the duplicate layer in LAB as in Part 1, but leave the Amount set to 500. In other words, don't complete the final step.
  3. Convert the image to RGB.
  4. Make an additional duplicate layers. For now make the top layer invisible and the middle layer visible.
  5. Set the blending mode of the middle layer to Darken.
  6. Tune the opacity of the dark halos with the opacity slider of the middle layer.
  7. Set the blending mode of the top layer to Lighten and make it visible
  8. Tune the opacity of the light halos with the opacity slider of the top layer.

Walk through

I started out by following my basic recipe for tuning USM Threshold and Radius as described in Part 1, with two exceptions:
  1. I made a duplicate layer and sharpened it, and
  2. I left Amount at 500.
When I was done, my USM dialog looked like this:


And my image looked like this:


This looks pretty bad, but don't worry. The sharpening opacity is just too great an about to fix that.

Next I moved the image into RGB, made an additional duplicate layer, made the top layer invisible, and set the blending mode of the middle layer to Darken. Like so:


At this point the image looked like this:


Now there are no light halos at all and the dark halos are as dark as they can be. It's easy to see here how much the light halos contribute to the over sharpening of this image, especially in the tree branches. Look closely at this image, though, The dark halos aren't as noticible as the light ones, but at 100% opacity, they are very evident.


I turned do the opacity slider of the Darkenlayer until the halos nearly disappeared and that backed off so that I could see their effect. I ended up at 35%. This will definitely vary depending on image.


At this point a 100% zoom looked like this:


At this point the full image looked like this:


A strange effect without the light halos, eh? But interesting to see just the dark halos. Usually the light halos so overwhelm the dark ones that they are all we see.

I set the blending option for the top layer to Lighten and made it visible:


It looked bad. The 100% light halos overwhelmed the image and it looked virtually identical to how it looked before I tuned the dark halos.But now I could complete the tuning by turning down the opacity of the light layer until the image looked good. I used the layer eyes to compare the image with and without the light halos. I arrived at a value of 12% for the opacity of the light layer. I often find that the right opacity for the light halos is about 1/2 that used for the dark ones, but again this is image dependent.


At this setting the image was clearly sharper, but the halos were not visible. The tree branches which would have made such a mess if I'd used the same opacity for light and dark halos now look OK.The result was the after image from the top of the tutorial.

One nice thing about this approach is we aren't really done until we flatten the image. The layers are still there and we can make them visible and invisible and tune their opacity.

Advanced variation

Not that advanced, really once you've digested the basic technique of having separate layers for light and dark halos. Why limit ourselves to the same radius for the light and dark halos? Duplicate before USM and use different Radius and even Threshold values for each of the two layers. This takes a little practice, since you won't actually be able to tell how it's all going to look until you put it together. But it can often be invaluable. A smaller Radius for the light halos in the above image might have been a good thing because the light halos live in the fine tree branches and the dark halos live in the far coarser stonework on the bridge.


The idea of sharpening is to get away with exactly as much as possible but no more. Often the light halos are the limiting factor. By separating them, we can use much higher values for the dark halos, creating a far greater illusion of sharpness without being detected. (And you thought it was just that great new lens, eh, mate?)
If not now, when?


  • ruttrutt Cave canem! Registered Users Posts: 6,511 Major grins
    edited April 14, 2005
    I fixed a simple but very basic problem in the description of the procedure. You have to make a duplicate layer before sharpening and sharpen it. It's very important to have the original unsharpened image as a background!
    If not now, when?
  • cletuscletus Master of Craposition Registered Users Posts: 1,930 Major grins
    edited April 15, 2005
    Great work, rutt thumb.gifclap.gif
  • ginger_55ginger_55 Crazy Creek Babe Registered Users Posts: 8,416 Major grins
    edited April 15, 2005
    You all, I plan to do this, just not today that I know of. I am very excited about the second part of this, but my body kind of shut down after stressful doctor visits yesterday, etc.

    And I am so excited that in one yr from June my daughter and family are moving to south carolina, 2 hrs from me. It is about all I can think about. I keep thinking I am going to announce it here, but haven't decided on people or wide angle. Sara, daughter, her husband and two grade school grandkids. I just can't concentrate on sharpening. I am going to do it, though. I am not uninterested. I am very interested.

    After all is said and done, it is the sweet tea.
  • DJ-S1DJ-S1 Life is good! Registered Users Posts: 2,303 Major grins
    edited April 15, 2005
    rutt wrote:
    I fixed a simple but very basic problem in the description of the procedure. You have to make a duplicate layer before sharpening and sharpen it. It's very important to have the original unsharpened image as a background!
    Oh good, I thought I was just being extra dumb! I couldn't see any difference the other way and I thought I was doing something wrong. Thanks for the tutorials, I know they take a lot of time to put together. thumb.gif
  • John MuellerJohn Mueller Long Shots Registered Users Posts: 2,555 Major grins
    edited May 2, 2005
    Thanks Rutt.
    I found this process a while back, but misplaced it.
    Now I have it againthumb.gif
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