Selective Sharpening Using High Pass in Adobe Photoshop

Sharpening has always been somewhat of a mystery to me. I've been taught that it should be done routinely with digital images and I've read a number of recipes for how to do it. I can see the changes that sharpening makes in an image, but I've never really understood why it's necessary or how much sharpening to apply. 
I do know what I like when I see sharpening changes and I know that I'm enough of a perfectionist that I want to have control over the amount and placement of the sharpening. For these reasons, my favourite method of sharpening uses a high pass layer and masks. This method of sharpening is infinitely flexible, and allows me to be as precise as I want while giving me wide latitude to change my mind.
This tutorial provides you with a bit of background about sharpening and why we use it, and takes you through the steps to selectively sharpen in Adobe Photoshop with a high pass layer and mask.
The need for sharpening digital images is a result of how information is digitally recorded. When an image is captured digitally, information is recorded in bits of data we know in photography as pixels. Each bit of data or pixel can only contain a single piece of information. Because a pixel can only hold one single piece of information, any detail that is smaller than a pixel is lost. Even the most subtle of transitions exceeds a pixel's ability to record one piece of information. 
If a transition between details happens within a pixel, the camera has to decide what single value will be recorded. The camera could direct that one of the two or more pieces of information is recorded, or it could direct the pixel to record one value that approximates the combination of pieces of information. The result in either case is loss of detail. Even at high resolutions, which offers more pixels for recording information, the total number of pixels still cannot come anywhere close to capturing and replicating the transitions that create the thousands of shades of even one colour. 
All digital cameras are programmed to detect edges — transitions from one value to another — then use digital sampling to create in-between values that approximate the combination of values in the transition. So, instead of recording the precise change in values that happens along edges, the pixels create a more gradual transition between values. Our brains see this information as soft edges or even a loss of focus.
Most digital cameras also use a filter over the sensor to blur light slightly. This "anti-aliasing filter" spreads information about small details over a number of pixels. This helps to avoid artifacts and patterns that digital recording tends to create with small details. However, the filter also contributes to softening edges.

How digital recording softens edges
These two samples show how pixels approximate information when a transition occurs within a pixel. You can see how what might have been a sharp edge will begin to look soft.

Sharpening is a process that corrects soft edges. It makes the dark side of an edge darker and the light side of an edge lighter. The result is more contrast between the two abutting edges. Our brains interpret this information as sharpness. 
With film, sharpening involved making a sandwich of the original negative and a faint positive image of the negative (a positive mask) with a piece of glass in-between. Because the glass placed the positive mask at a slightly different distance from the enlarger's lens, the positive mask would be slightly out of focus. The slightly out of focus mask — the unsharp mask — caused halos to form around the edges in the final photograph. Those halos made the dark side of an edge darker and the light side of an edge lighter, which, as I've mentioned, our brains see as sharper. This process was called unsharp masking, thus the name of one of Photoshop's sharpening tools,Unsharp Mask.
In order to correct the softened edges caused by digital recording, we use post-production image editing to lighten a column or row of pixels on the light side of an edge and darken a column or row of pixels on the dark side of an edge. We control the number of pixels that will be affected and how much they'll be affected by manipulating the settings in the various sharpening tools.

How sharpening works
These images show the effect of sharpening (the top left is unsharpened and the bottom left is sharpened) and how the sharpening happened (the light edge is made lighter and the dark edge is made darker).

Because softness is inherent in digital image recordings, it's common to hear or read that you should sharpen all digital images. But if we increase the number of pixels on a sensor, more bits of information can be recorded, meaning there's more detail in the image. Thus, higher resolutions reduce the need for sharpening or at least reduce the amount of sharpening needed. As a result, newer commentary suggests that sharpening may no longer be a routine consideration. Instead, sharpening is an elective process used when the image needs it.
Sharpening can also be done for creative effect. Primarily, sharpening will make fine details more noticeable. 

Sharpening to bring out details

Because sharpening adds contrast in fine details, it can be used to enhance texture in an image and add a sense of clarity. Sharpening is commonly used on eyes in portraits for this purpose in order to give the subject a bright-eyed look. 

Sharpening eyes in portrait

Sharpening can also add depth and focus to certain areas of an image so it may be used to highlight objects in a photograph such as in a still-life. (We will see that effect with the image image used in the instructions below.)
Sharpening is an art, not a science. First, consider the content of your image. You likely don't want to add sharpening where you want smoothness or gentle transitions. For example, sharpening is typically not applied to a person's skin, patches of mist, or big fluffy clouds. Some images such as photos of babies and children even benefit from the inherent softness. In contrast, you likely will want to apply sharpening to enhance texture and bring out details. For example, judicious sharpening can make a person's eyelashes more noticeable or bring out the spidery nature of veins in a leaf. Landscapes typically benefit from more rather than less sharpening, as do animal portraits in which you want to emphasize details such as feathers or hair.
The goal is to apply sharpening where you need it and apply enough sharpening to make edges sufficiently pronounced while also minimizing the halos that sharpening creates around edges.
The second consideration is how you will output your image. Large prints on fine art paper viewed from a distance will require more sharpening than small prints on bright white lustre or glossy paper viewed in an album. Similarly, images projected onto a large display will require more sharpening than those posted on the web or sent in email.
I've seen calculators and formulas for determining how much sharpening is required for different outputs. However, I find that all sharpening has to be evaluated by eye and at the best, a calculation or formula can only give you a rough place to start.
As with almost every image editing technique, sharpening can be overdone. In fact, it's very easy to do so! If you think about the analogue version of unsharp masking, you'll understand how the noticeable appearance of halos in your image is a sign that you've taken sharpening too far. Another sign of over-sharpening is a loss of gradation in colour. Too much sharpening will make the colours in the sharpened areas look clumpy.

The image on the left is not sharpened. By sharpening it a bit (middle), the details become clearer. The individual grapes are more distinct, for example. Once over-sharpened (right), the colours lose their natural transitions and break down into clumps. The grapes no longer look as plump and the speckling on the pear looks unnatural.

Sharpening can't recover details lost to poor lighting and it won't help to focus an image if the image is out of focus to begin with. In fact, sharpening an out of focus image will bring contrast and clarity to the parts of the image that are in focus, making the blurriness even more noticeable. Sharpening will also make areas of lost details appear hazier in comparison. If there is some focus - some edges - in the image, sharpening may tighten the focus or give the impression of improved focus.
There are hundreds of suggestions, tips, and methods for sharpening images. My favourite technique for sharpening uses Photoshop to create a high pass layer and then layer masks to control where and how much sharpening is applied.
A high pass layer works in the same way an unsharp mask works on film: the high pass layer creates slightly blurry halos around edges in the original image, thus increasing the contrast at edges. The difference between high pass sharpening and most other methods of sharpening is that high pass sharpening does not actually adjust or change any pixels in your original image. Also, because high pass exists as a separate layer, you can adjust the layer's Opacity and Blending Modes to control the strength of sharpening over the entire image. You can then use masks to control where sharpening is applied within the image and to make localized changes in the strength of sharpening.
Another advantage to sharpening with a high pass layer is you can save the layer with the Photoshop file and go back later and change the settings and areas where sharpening has been applied. This is a huge advantage when working with an image that you might finish for web viewing and printing, or printing on different papers.
Let's get started. 
I'm going to sharpen the image below. I want to bring out the details in the back left corner (in the circle) and prevent the details in other places (at the arrows) from becoming overly sharp. If I let the artwork at the back of the window display become too sharp, the background will distract from the foreground. If I let the edges of the eyes in the centre mask become too sharp, the mask will look painful to wear. Overall, I'll need to prevent the colours from clumping and the details from breaking down as a result of too much sharpening.

Assess  your image to determine what needs sharpening

Sharpening is usually the last or almost the last thing you do when preparing an image, and you want to be sure that you are applying sharpening to a finished, complete image. Therefore, before making your sharpening layer, ensure you have a duplicate layer of your completed image. 
If your work is all on one layer, then just duplicate that layer (Command-J/Control-J). If, however, you've been working on a few layers, then merge the layers together into anew layer. In Photoshop, this is called Stamp New Layer
To Stamp New Layer, check that all layers you want included in your final image are visible (turned on). Click on (select) your topmost layer and then stretch your fingers to use Command-Alt-Shift-E/Control-Alt-Shift-E to activate the Stamp Visiblecommand and make that stamp a new layer. (There is no menu item for this command.)
You now have a flattened version of your final image while preserving all of your independent layers. In addition to allowing you to sharpen without making changes to your image, creating this layer allows you to still access your original layers to make further changes, should you wish.
To keep your work organized, rename your new merged layer "High Pass Sharpening."

Create a copy of your final image in a new layer

A disadvantage to sharpening with a high pass layer is the potential for increasing or adding noise to a photo. With other sharpening tools such as Unsharp Mask, you can control noise problems with adjustments to the different values set in the tool. With the high pass option, you control noise by doing three things:
  • Ensure your image capture is as clean as possible. This means using a tripod if not shooting at a high shutter speed, choosing the lowest ISO possible, and using the correct exposure.
  • Deal with any noise while processing your image. (If you're not confident about reducing noise in your image, check out our series "How to Use Noise Reduction for Silky Smooth Photography" and in particular, Marie Gardiner's article "How to Use the Reduce Noise Filter in Adobe Photoshop.")
  • Desaturate the high pass layer. Even when working on a colour image, the colour information in a high pass layer is irrelevant, so we're going to remove that information right from the top to ensure that extra information doesn't add noise. 
Because we don't need to preserve an adjustment layer, desaturate the high pass layer by going to Image > Adjustments > Desaturate or use the shortcut Command-Shift-U/Control-Shift-U.

Desaturate the high pass layer

To apply the high pass filter to your sharpening layer, go to Filter > Other > High Pass
This will bring up a dialogue box with a Radius slider. You want to increase the Radiusslider (increase the number of pixels affected) until the details in the image just begin to pop. You'll find you need more Radius when you're working with high resolution images. Don't be surprised if you need around 10 to 20 pixels or more. Select OK.

Adjust the Radius slider until the details in the image begin to pop
The top image has not been sharpened enough; the details appear to be behind a veil. The middle image has been sharpened too much; large patches of colour on the centre mask - which are not edges - are popping. The bottom image has been sharpened correctly for the area that needs the most sharpening (the back left of the image). In that area, the details are just beginning to pop through the layer of grey.

Change the layer's blending mode in the blending options drop down menu to Soft Light. As you become more familiar with using high pass sharpening, experiment withHard Light and Overlay blending modes as well.
You will likely find that you now have a bit more sharpening than you need. That's okay; starting with 100%, adjust the Layer Opacity down to get the amount of sharpening you need. Aim for an opacity that gives you the right amount of sharpening in the area of your image that needs the most sharpening.

Step 7: Mask the Layer As Needed
Add a layer mask by clicking on the Layer Mask icon at the bottom of your Layerswindow, or by going to Layer > Layer Mask
Depending upon how much sharpening you're going to need, you have three options:
  • If you need sharpening in many places, add a white layer mask (Reveal All or click on the Layer Mask icon a the bottom of the Layers window), then paint out what you don't need with a black brush.
  • If you need sharpening in just a few places, add a black layer mask (Hide All orAlt-click if you're adding a mask with the Layer Mask icon) and paint in what you need with a white brush.
  • If you will both add and take away sharpening, add a white layer mask (Reveal All or click on the Layer Mask icon a the bottom of the Layers window), then fill the mask with 50% grey. (Set the Foreground colour to 50% grey and with the layer mask active, use Alt-Delete to fill the mask.) Paint in what you do need with a white brush and paint out what you don't need with a black brush.
To begin adding or removing sharpening, ensure your colours are set to the default Foreground and Background (D). Use a very soft brush (0% Hardness) at a medium or larger size. Paint in or out sharpening as you need. Change the Opacity of your brush to control how much sharpening you paint in or out.
Turn on and off visibility of the high pass layer while you work in order to check the changes you've made. It's better to not apply quite enough sharpening than to over sharpen an image. Avoid creating edges that become unnaturally razor sharp or causing colours to clump. The dreaded sharpening halos are not as obvious with high pass sharpening as they are with other sharpening tools, so you may not see them before you notice the other two changes. 

High pass sharpen mask
I chose to fill my mask with 50% grey and then both paint in and out sharpening as I needed. As I finished, I realized that I wanted a bit more general sharpening across the whole image, so I lowered the opacity of the layer mask, effectively applying more sharpening over the whole image.

If you discover after you've sharpened that you need to make other changes in your image, you can still work on the adjustment layers beneath without having to redo the sharpening. You will only have to redo the sharpening layer if you change the actual content of your image by, for example, cloning.

Make final changes in image adjustment layers if necessary
Once I finished sharpening my image, I concluded that my image was slightly oversaturated, so before outputting my final image, I made an adjustment in the Hue/Saturation layer that was beneath the High Pass Sharpening layer.

Digital images often need sharpening because the process of recording a digital image softens edges and details. There are many approaches to sharpening but I like using a high pass layer with a mask. This method of sharpening adds a layer to the image and all changes are made on that layer; therefore, high pass sharpening does not change the original image. The layer's blending mode and opacity can be adjusted and changed when and as much as needed to fine-tune sharpening over the whole image. By applying a mask to the sharpening layer, sharpening can be further refined by selectively applying it only to certain parts of the image and, if desired, in varying amounts. Finally, a high pass sharpening layer allows you to see the effects as you work so you can judge in the process what changes need to be made.
Sharpening and noise reduction go hand-in-hand when preparing a final image. Marie Gardiner's article, "How to Use the Reduce Noise Filter in Adobe Photoshop," guides you through reducing noise in a smooth and straight-forward way.


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