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The High Pass Way to Sharpen in Photoshop

While the oddly named Unsharp Mask filter is the most common way of sharpening in Adobe Photoshop, there are other ways to do it as well. This week we'll look at something known as "High-Pass" Sharpening, then next week we'll cover some advanced sharpening techniques in Photoshop. Good sharpening skills can greatly improve your images, so it's worth spending a bit of time discussing the topic.

One of the downsides to traditional unsharp masking is that the changes are made directly to the image you are working on. I generally prefer making updates on Adjustment Layers, but Unsharp Mask doesn't work that way. Sure, you can duplicate the layer and sharpen it, leaving your original image untouched. But this doesn't mean the effect itself is on a layer, only that you have two copies of your image, one sharpened, and one not. If you want to alter the amount of sharpening, either overall or selectively, you have to start over and resharpen.

A good alternative is to use a technique known as High-Pass sharpening. It is a bit more complicated than unsharp masking, but not by that much. You won't find it listed on the menus either so you'll have to remember the steps involved (or just bookmark this page of course).

First, we'll need to duplicate your image layer. I know I said that this wasn't a good approach with Unsharp Mask, but just bear with me. Things will work out better with the High-Pass method, I promise. If your image is on a single layer already, you can simply drag that layer to the "new layer" icon at the bottom of the Layers pallet or, if you prefer, right click on the icon for that layer in the Layers pallet and select "Duplicate Layer..." from the pop-up menu. If you have already made multiple layers to build your image and don't want to merge them, you can select the entire image and use "Copy Merged" followed by "Paste" from the Edit menu. Regardless of the method you choose, you want the new layer to be on top of the image layer(s).

Next, change the Blending Mode of this new layer to Overlay. After you have experimented with this technique for a while, you may want to try "Hard Light" and "Soft Light" Blending Modes instead, but for now we'll stick with Overlay. Regardless of which you use, the image will get noticeably darker and the contrast will go through the roof. These are temporary effects that will vanish in later steps so don't panic. Remember, too, that all this is on a new layer so you can throw the whole thing away and go back to your original if you don't like the way it comes out.

As a third step, we'll need to apply the High Pass filter for which the technique is named. You can find it at Filter >> Other >> High Pass and you'll be glad to know it works in either 8-bit or 16-bit modes. With the image set to a full 100% "Actual Pixel" view, set the radius slider in the High Pass filter dialog to a value that makes the image look just a bit too sharp. The exact value isn't critical since the next and final step will let us fine tune things. For web images, you should find this to be around one to two pixels, whereas images sized to print could easily require a radius of ten or more pixels. Note that while your image itself should start to look relatively normal at this point, the preview image in the filter dialog will look rather odd. It should be mostly gray with detail only showing where the image has edges. Indeed, if the preview doesn't look mostly flat gray, you have the radius set too high. Click on "OK" when done. Remember, since we'll be fine tuning things in the next step, so you can safely set the radius such that the edges are slightly too sharp now, without fear that you'll have to live with it being that way when we're done. For now, you want the preview image to look like a flat gray embossed version and the main image to look normal, but just a bit too sharp. This will get you in the right range and still give you a bit of headroom in the effect if you actually decide you do want more sharpening later.

Step 1: Duplicating the image layer
Step 1: Duplicating the image layer
Step 2: Setting mode to Overlay
Step 2: Setting to Overlay mode
Step 3: Running the High Pass filter
Step 3: Running the High Pass filter

As the last step, use the Opacity slider on the High Pass layer in the Layers Pallet to adjust the effect as desired. If you've set the radius correctly above, you should be able to go with an opacity around 50%, but don't worry if you need considerably higher or lower values. Twenty-five to seventy-five percent is not uncommon.

In Overlay, Hard Light and Soft Light blending modes, medium gray has no effect on the underlying image. Since we set the High Pass filter so that the preview image was mainly flat gray, the choice of blending mode has the "fringe" benefit of thereby concentrating the sharpening effect only in the areas where detail did show — the edges. If there are parts of the image such as areas of open sky that you want to be sure don't get any sharpening, you can use a layer mask (something I'll be talking about more in the weeks to come) or paint on the High Pass layer using medium gray. Not sure how to get medium gray? No problem: open the Color Picker and set the Red, Green and Blue values to all to 128.

Also, a layer in any of these blending modes only modifies whatever is beneath it; it does not completely cover it as "Normal" blending does. You can still make a reasonable degree of touch ups to the image itself without affecting the effect of your sharpening. If you make changes that alter any edges though, you'll likely need to recreate your High-Pass layer.

Once you get the hang of things, you should be able to go through this series of steps fairly quickly, but as with many repetitive tasks in Photoshop, you can always record an Action and just play it back when you need to sharpen an image.

Step 4: Adjusting Opacity
Step 4: Adjusting Opacity
The final sharpened result
The final sharpened result

Sharpening is a necessary evil in digital editing. I'm not sure I can say that High-Pass sharpening produces better results than unsharp masking (they both give great results), but it does let you sharpen on a layer, something that can be a prudent move as it lets you retain the maximum degree of control over your underlying image.

By the way, for the curious, the screen shots for this week's Phototip were done using the brand new Photoshop CS2. I'll probably have some thoughts on the new release in the weeks to come.


Date posted: May 1, 2005

 

Copyright © 2005 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: Behind the Unsharp Mask: The Secret World of Sharpening Return to archives menu Next tip: Advanced Sharpening in Adobe Photoshop

Related articles:
More Than a Bit of a Difference: 8-bit Versus 16-bit
More on Photoshop Levels
Behind the Unsharp Mask: The Secret World of Sharpening
Sharpening in Adobe Lightroom 3
The Best Way to Fix Over-Sharpening
Sharpening is an Optical Illusion
 

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