HDR Cookbook - Sharpening

Sharpening is a vital step in post-processing. Especially the HDR process tends to soften images. Most people use the Unsharp Mask filter to sharpen their images. While this usually works acceptably well, it has an important drawback: it is destructive. That is, is changes the actual pixels of the image layer it is applied to. Thus, you cannot easily undo it or change its intensity after you applied it. More importantly however, it cannot be easily applied to images that consist of blended layers (where different parts of the image are on different layers which are blended together using layer masks. In this case, you would have to apply the unsharp mask to each layer separately.

There is a quick an easy way around this using the High-Pass filer in Photoshop.

Requirements and Assumptions

1. I assume that you have one layer that represents your image. You will need to copy this layer and apply the high-pass filter to it. If your image data is dispersed over multiple layers (e.g. two layers blended into each other using layer masks, then you will have to reduce these to a single layer which you can use for the sharpening. Select the layers in the Layers palette, right-click on one of them and select “Merge layers” from the popup menu.
2. Any kind of retouching has to be completed before you do the sharpening! If you do the sharpening before the retouch (e.g. removing unwanted objects of artifacts) that shadows of these objects will remain in the sharpening layer and you have to redo the sharpening.

Overview

The whole process is rather simple:

1. We make a copy of the image layer and set the layer mode to Overlay
2. We apply the High-pass filter to this layer

Remarks before we start

Setting the zoom: Before you start applying any sharpening (no matter which method you use) set the zoom level to 100%, 50% or 25%. Any zoom level in between these will cause interpolation artifacts on the screen that will make it hard to judge the result.

Sharpening for print or screen (Internet): It is well-known that you need to sharpen more aggressively when you want to print the resulting image as opposed to when you want to have it displayed on-screen. Keep that in mind while you sharpen the image. How much more aggressively you have to apply the sharpening depends on the print size and other factors. A bit of experimenting is necessary here. As a rule of thumb: If it looks good on the screen, you may need to ramp it up a notch for printing to a point where you would say it is a bit too much.

Detailed Process

1. Create a copy of the image layer. We will call this layer the “sharpening layer”. If you apply noise reduction, you should sharpen after the noise reduction step, and you should use the layer on which you reduced the noise. This ensures that your sharpening will not amplify any noise artifacts but only the edges in the image.
2. Set the new copy’s layer mode (drop-down menu at the top of the Layers palette) to “Overlay”. You image will suddenly have an unnaturally high contrast. Don’t worry this will disappear.
3. Position the sharpening layer before all other layers in the layer stack that carry relevant information.Only those pixels below the sharpening layer will be sharpened.
4. Select the new layer copy and choose “High-Pass…” filter from the “Other” section of the “Filters” menu.
5. In the dialog that appears set the slider to a value between 2 and 4, depending on the image content, the image size and your preferences. If you check the preview box in the dialog, you can see the effect directly in the image.
6. Click Ok when you’re done.
7. You can adjust the intensity of the sharpening at any time by setting the opacity of the new sharpening layer.

Selective Sharpening

Some images require selective sharpening where some parts of the image (e.g. the bricks in a wall) are sharpened while others (e.g. the sky) remain untouched. This can be easily achieved with the high-pass filter method as follows:

1. Create a layer mask on the sharpening layer.
2. When you only want to sharpen small regions, Invert the mask by selecting it and pressing Ctrl-I. The mask is now completely black.
3. Use the white brush to reveal any part of it to sharpen the image in those areas.

A variant of this selective sharpening is the following:

1. Select those parts of the image that you want to sharpen using any of the selection tools available to you.
2. Select the sharpening layer and create a layer mask from the selection.
3. You may want to apply a slight blur to the layer mask (e.g. with a Gaussian Blur filter with a radius between 2 and 5) if the sharpening has unnaturally harsh edges.
   

   Two selective sharpening layers - one for the chains (with a group mask) and one for the arches (with a layer mask)

Selective sharpening can also be easily combined with a general sharpening by having one global sharpening layer as explained at the beginning of this section and one or more sharpening layers for specific areas of the image with respective layer masks. Fine-tine the overall result by changing the opacities of these layers.

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