Sharpening digitally captured images in Photoshop, or any other editing software is unavoidable because of the softness introduced in the digitizing process no matter how good the capture device may be. When we digitally photograph the continuous tone real world we force the view to be broken into pixels. This brings with it the potential to introduce stepped diagonals, interference patterns, and other digitizing artifacts. To combat this problem, all capture devices use a process called aliasing through which suitable intermediate tones are added to where there are “edges” in the photograph. This alone introduces some softness to the image. Additionally, most cameras use moiré filters to minimize the interference patterns that could be distracting in the finished image. So, we start with a slight handicap of softer than real captures. Depending on the in-camera settings, some “sharpening” may be applied by the camera or in the case of RAW image captures nothing is added. Therefore we need to inject some level of sharpness to the images that are lost in the process of photographing.
Different software handles this in slightly different ways. But, the fundamental principle of sharpening is as follows:
- There is an “edge” when a lighter tone abruptly changes to a darker tone
- The “sharpening” process lightens the lighter side and darkens the darker side
- Thus increasing the edge contrast creating the illusion of sharpness
Sharpening is a carefully controlled workflow that consists of capture sharpening, creative sharpening, and output sharpening. Although each stage may need different strengths of sharpening applied to different parts of the image, their fundamental principle remains the same as explained above. If applied carelessly, sharpening can introduce unwanted halos around the edges detracting from the overall quality of the photograph. So, the process should be such that we introduce halos everybody sees but nobody notices. In other words, the illusion remains enchanting but elusive.
The main sharpening tool in Photoshop has been the Unsharp Mask for many generations in the software. It has improved over time but the main parameters remained the same. USM is applied by Radius, Amount, Threshold which control how wide the halos will be, how strong the contrast between the dark and the light sides of the edge will become, and how much difference between the two will trigger the contrast enhancement. As I mentioned earlier, the light halos visible at the junction of lighter tones with darker tones are far more noticeable than the dark halos which seem to magically disappear into the shadows, literally!
In a straightforward application of the USM filter, reducing the light halo will need one or more of the following:
- Reduce the radius
- Reduce the amount
- Increase threshold
All of these, alone or in combination will reduce the effect of sharpening, perhaps to the point of defeating its purpose. The method I will explain in this post will allow controlling the white halos separately from the dark halos. I call this “Split Sharpening”.
Take a look at the original and the sharpened images below. Click to enlarge and use the arrows to see one image after the other.
The white halos along the edge of the wind turbine wing, the building roofline, and the wall are clearly visible. I applied USM with a radius of 1 and an amount of 70. I could lower the radius and/or the amount which will reduce the white halos but will not totally eliminate them.
To combat this problem I will separate the lightening function from the darkening function into two separate layers. First I will explain how to do this manually then I will provide a download link to a Photoshop action which will do this for you automatically.
My layers palette contains two layers, the original background layer and a sharpened image layer titled USM and shows the parameters of sharpening. Follow the simple steps below to enter the comfort zone of Split Sharpening:
- Target the USM layer and change its name to USM R=1 A=70 Lighten
- Change the blend mode of this layer to Lighten
- Press Ctrl-J to make a copy of this layer and change its name to USM R=1 A=70 Darken
- Change the blend mode of this layer to Darken
Now, you can do the following to control the white halos:
- Lower the Lighten layer opacity
- Add a layer mask to the Lighten layer and mask the areas to eliminate halos
- Do both
Compare the sharpened image above to the one below. It is the same sharpened layer simply split into lighten and darken layers and some areas masked as shown in the mask image.
And the layer stack now looks like the image on the left.
Now, let’s talk about the Photoshop Action to split a sharpened layer into lighten and darken components. It is nothing more than the steps above captured in the action which plays the steps back in the same sequence. Download the Ekin Sharpen Action and save it to a location on your computer (right-click on the link and choose to save it).
Then follow the steps below:
- UnZIP the file
- Copy it to where you keep the Photoshop actions. This, on a Windows system, is
C:\Users\Your Name\App Data\Roaming\Adobe\Adobe Photoshop CS6\Presets\Actions
But the uncompressed action file can be anywhere so long as you know where it is
- Start Photoshop, open Actions palette if not open
- Click on the panel menu icon on the top right corner of the panel and choose Load Actions
- Navigate where you saved the Split Sharpen action and click on OK
- From now on, you can make a layer to sharpen, apply unsharp mask the way you normally do, and then select this action and click on the play icon at the bottom of the Actions palette.
- It will create a group with two layers with lighten and darken blend modes and adjust their opacity to 50% which you can change to suit the image needs and mask either layer to hide the effects of sharpening.
As the title implies, this is part 1 and there will be at least part 2 but there may even be more posts and actions coming on this subject. If you do not normally receive new post alerts you may want to do so using the Subscribe for New Posts.
Here are the full-size images showing the original, sharpened, split sharpened, and the lighten layer mask hiding the halo producing edges.