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.
What is Image Sharpening?
To understand why the High Pass filter is such a great tool for sharpening images in Photoshop, we need to understand how image sharpening actually works. After all, we don’t get in there and physically sharpen pixels the way we’d sharpen a set of knives or a pair of ice skates. So what does “image sharpening” even mean?
Well, much like any good magic trick, image sharpening is nothing more than an illusion. It works by increasing contrast along the edges of objects in an image. Of course, Photoshop has no way of recognizing specific objects, so it considers an edge to be any area where there’s a big, sudden change in brightness or colour between neighbouring pixels.
When you increase contrast along the edges, making the light side of the edge lighter and the dark side of the edge darker, your brain sees the increased contrast as “sharper”. The more we boost the edge contrast, the sharper the image appears. But really, image sharpening has nothing at all to do with “sharpening” pixels. It has everything to do with increasing edge contrast.
Why Use The High Pass Filter?
So now that we know that we sharpen images by increasing contrast along the edges, what does that have to do with Photoshop’s High Pass filter? Well, before we can increase contrast along edges, we first need a way of detecting the edges themselves. That’s where the High Pass filter comes in. High Pass is an edge-detection filter. It looks specifically for edges in the image and highlights them. Any areas that are not considered part of an edge are ignored. Once we have the edges highlighted, we can then combine the results from the High Pass filter with one of Photoshop’s contrast-boosting blend modes (as we’ll see a bit later on) to easily increase contrast along edges without affecting the rest of the image.
Let’s get started.
Step 1: Evaluate Your Image
I am going to sharpen the image below. I will sharpen the bird and parts of the branch and prevent details in other places.
Step 2: Stamp New Layer or Duplicate Your Image Layer
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 a new 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 Visible command 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 organised, rename your new merged layer “High Pass Sharpening.”
Step 3: Desaturate the High Pass 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 can control noise either by doing two 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.
Or, you can just 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.
Step 4: Apply the High Pass Filter
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 Radius slider (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. Select OK.
*Note: The Radius value controls the amount of highlighting that’s applied to the edges, but that’s really an oversimplification. What the Radius value actually does is it determines how many pixels on either side of an edge that Photoshop should include as part of the edge. A Radius value of 1 pixel, for example, would mean that Photoshop would include only a single pixel on either side of the edge; one pixel on the light side and one pixel on the dark side. If we increased the Radius value to, say, 10 pixels, then Photoshop would extend the width of the edges to 10 pixels on either side. I recommend keeping the value between 3-4 for high resolution files and 0.3 to 1.3 for low resolution files.
Step 5: Set the Layer’s Blending Mode
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 with Hard Light and Overlay blending modes as well.
*Note: The Overlay blend mode ignores any areas of neutral gray, so all of those non-edge neutral gray areas created by the High Pass filter instantly disappear from view. It then uses the lighter highlights to lighten the light sides of the edges even further, and the darker highlights to darken the dark sides of the edges even further, boosting the contrast of the edges and creating the illusion of a sharper image. If you find that the sharpening effect you’re getting from the Overlay blend mode is too strong, try the Soft Light blend mode instead. It works exactly the same as Overlay but the results are more subtle. Or, if you find that the Overlay sharpening effect isn’t strong enough, try the Hard Light blend mode, which will give you the most intense sharpening of the three.
Step 6: Adjust the Layer Opacity
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
This step is for advanced users.
Add a layer mask by clicking on the Layer Mask icon at the bottom of your Layers window, 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 at 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 or Alt-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 at 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.
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.Flatten the image before saving.
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.