SELECTIVE SHARPENING
HANS WEICHSELBAUM RUNS US THROUGH THE VARIOUS ADAPTATIONS POSSIBLE BY UTILIZING THE HIGH PASS FILTER
Even after many years of working with Adobe Photoshop, you’ll stumble across tools and filters you’ve never seen or heard of before. Many of these are specialized gadgets that you’ll rarely use, but there are also useful filters — some of which you may not even know that you need.
The High Pass filter is one of the more familiar in the group of obscure filters and is widely used to mitigate the softening effects of the camera’s antialiasing filter. While not technically a sharpening filter, it can produce results that are as good as, or even better than, Photoshop’s actual sharpening filters — while, at the same time, being just as easy to use. But, there’s more! As contradictory as it sounds, you can also use this filter to soften portraits and to enhance mid-tone contrast, too.
The High Pass filter is not the type of filter that you simply apply to your images like most other filters. To see what it does, open an image and run it through the filter (Filter > Other > High Pass). You’ll be greeted with an image that has a silver-grey emboss over it. In most cases, the High Pass filter is not applied directly on the image but instead to a duplicate layer sitting on top of the original. Then, the blending mode is changed to one member of the Overlay family of blending modes (Overlay, Soft Light or Hard Light). The effect you get will depend on your combination of Radius setting, blending mode, and the top layer’s opacity setting.
HIGH PASS SHARPENING
Manual sharpening is an essential part of processing any image, though it differs greatly according to the image. Images with lots of fine detail will need a different kind of sharpening than will images with no sharp edges (think of the soft, moody quality of a foggy landscape, as opposed to the striking lines of urban architecture, for instance). Sometimes, you need selective sharpening — for example, a portrait should have sharp eyes, but you probably won’t want to highlight wrinkles and every skin imperfection. Again, the optimum settings will depend on the size of the image and its final output — images for monitor display demand a very different amount of sharpening than do large prints. Sharpening, therefore, should always be the last step in the editing process.
Ideally, a small amount of sharpening should be applied during RAW conversion to compensate for the effect of the Low Pass filter, also called the ‘antialiasing’ (AA) filter.
The standard tool for sharpening was (and still is) the Unsharp Mask filter, while High Pass sharpening is something totally different. This filter highlights edge detail and has a lot in common with the Find Edges filter (Filter > Stylize > Find Edges).
High Pass sharpening is particularly helpful if an image doesn’t contain well-defined sharp edges that the Unsharp Mask filter can latch onto. Animal photography can often fall into this category, especially shots taken underwater. Compare Image 2 with the High Pass sharpened version (Image 3).
The Unsharp Mask filter would have required a very high Radius setting to produce a similar sharpening effect; the result showing very noticeable halos around the edges.
Sharpen using the High Pass filter by making a duplicate of the current image layer, then applying the High Pass filter to the duplicated top layer (Filter > Other > High Pass). I used a Radius setting of 10 for this particular image (Image 4).
You can see that all the flat areas come up as mid
grey, whereas the edges are accentuated. Now, let’s think about blending modes. Remember that the Screen blending mode lightens the lighter areas, while Multiply darkens the darker areas of the underlying image. Overlay is a combination of both; therefore, it lightens light areas and darkens darker areas. Mid-grey regions are not affected. If you change the blending mode of the upper layer to Overlay you will get the effect shown in Image 3. If the result is too strong, you can reduce the opacity of this layer. Another way of changing the amount of the sharpening is through changing the blending mode to Soft Light for a softer effect or to Hard Light for a more pronounced result.
A further variable is the Radius setting of the High Pass filter. For proper sharpening, it should be in the range of 0.5 to 3.0. As mentioned, I used a setting of 10 pixels for this image, which is not really a textbook sharpening but, rather, an increase in mid-tone contrast. A good way of seeing the effect of the Radius setting is to change the blending mode of the duplicate top layer to Overlay before applying the High Pass filter. This will allow you to see exactly what the Radius setting will do to the sharpening process.
The next image (Image 4) has lots of fine detail. Here I used a small Radius setting of around 1. The results are similar but not quite the same as with the Unsharp Mask filter. In the case of highly coloured images, it is also a good idea to set the saturation of the top layer to zero (Image > Adjustments > Hue Saturation). This eliminates any colour fringing around the sharpened zones.
SOFTENING AN IMAGE
The High Pass filter can also be used for softening an image and is especially effective for skin tones. The effect is similar to the old darkroom technique of holding a diffusion filter under the enlarger during the exposure.
Again, start by duplicating the background layer. Run the High Pass filter over the top (duplicate) layer, but set the Radius to a fairly high value, around 15. Next, go to Image > Adjustments > Invert to get a negative of the top layer, then change the blending mode to Soft Light. This will create a beautiful, ethereal soft-focus effect.
You can stop right there, but you might want to keep parts of the image sharp, such as the eyes and the mouth in a portrait. Image 5 was softened with the High Pass filter at a fairly high Radius setting of 20, with the blending mode set to Linear Light and the Opacity reduced to 30 per cent.
Here is an example of a portrait — the original (Image 6) compared with the softened version (Image 7). The High Pass filter with a large Radius of 30 was used, then the layer was inverted with a Soft Light blend setting, and set to 88-per-cent opacity.
The sharp eyes and lips and part in the hair were recovered from the underlying original with a layer mask. To do this, simply paint with a black brush onto the white layer mask. For more control over the effect, use a soft-edged brush and reduce the brush opacity. Here, for instance, I brushed a few times over the eyes to get the full original sharpness back, while the hair was only painted over once or twice in some places.
INCREASING MID-TONE CONTRAST
This is a third way of using the High Pass filter and is very similar to the sharpening technique, but it uses a fairly large Radius of more than 10. This now increases the contrast not along the edges but between larger areas. (See Images 8 and 9.)
If there is no distinct difference when using the High Pass filter at a medium or a large setting, it’s best to apply the blending mode first, before the High Pass filter. Then, you can follow the effect directly by changing the Radius setting.
The increase in mid-tone contrast can also be dramatically increased by using the Hard Light blending mode instead of Overlay. In contrast, the Soft Light blending mode will give you a more subdued effect.