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Photoshop CS's high pass filter is very versatile. There are three ways you can put it to use. One, it's a great sharpening tool. Two, contradictory as it sounds, you can use it to soften portraits and other photos. Three, you can use it to create a high contrast portrait effect. In this tutorial, I'll show you how to do all three.
Step 1. Sharpening
This is the image that I want to sharpen. The reason that it needs sharpening it is that all digital photos have a natural softness caused by the anti-aliasing filter located in front of the camera's sensor. The purpose of the anti-aliasing filter is to disperse the light that passes through it to avoid moire effects seen in clothing and other textiles. But it also softens the image, which is why you normally need to sharpen your photos.
Sharpening is always the last step in post-processing. This is because the amount of sharpening you need to apply varies according to how you intend to use the photo. A photo that's going to appear on the internet, for example, requires a different amount of sharpening to a photo that's going to be printed on an inkjet printer. I always save my photos unsharpened, so that I can apply the appropriate amount of sharpening when I know where the image will be used.
Right click on the Background layer and choose Duplicate layer.
Go to Filter > Other > High Pass...
Set the Radius. The higher the Radius, the more sharpening is applied to the photo. Somewhere in the range of 0.5 to 3.0 is about right, depending on the subject matter and how much you want to sharpen it.
Go to Image > Adjustments > Hue/Saturation... and set the Saturation to zero. This eliminates any colour fringing.
Change the Blending Mode to Overlay.
Here you can see the difference after sharpening. You may have to look closely to see the difference - the unsharpened photo is on the left and the sharpened one is on the right. The level of sharpening required is often subtle. Too much introduces artefacts into the image, normally seen along lines marking the edge of a dark subject against a light one, such as the horizon.
Step 2. Softening Skin Tones
The high pass filter isn't just for sharpening - it can be used to soften skin tones as well. The effect is similar to the darkroom technique of holding a diffusion filter under the enlarger during the exposure to soften the image. The advantage of doing it in Photoshop CS is that you can apply the softening selectively to the parts of the image where you want it.
This is a technique that's good for softening the skin in your portraits. The idea is to apply the softening effect to the entire image, then use a layer mask to uncover the parts of the image that you want to remain sharp - such as the eyes, mouth and hair.
This is our starting portrait.
Right click on the Background (Cmd-click with a Mac) and select Duplicate background. Click OK on the window that pops up. This creates a duplicate layer to which we will apply the High Pass Filter.
Go to Filter > Other > High Pass... and set the radius to around 15. You can experiment with this figure; the higher the number, the more you'll soften the image.
Go to Image > Adjustments > Invert to invert the layer and change the blending mode to Soft Light. This creates a beautiful, ethereal soft focus effect. If you're happy with this effect as it is, then you can stop right here. But the beauty of using Photoshop CS is that we can use a layer mask to uncover parts of the background that we'd like to show through. This gives us the option to sharpen up parts of the portrait that we'd rather be sharp than soft - such as the eyes.
Click on the Add layer mask icon at the bottom of the Layers Palette. This adds a layer mask to the active layer. The layer mask is shown as a white rectangle in the Layers Palette.
Select the Brush Tool. A soft brush is best for this technique as it avoids hard edges, so set the Hardness to 0%, the Mode to normal and the Opacity to 80%. Set the diameter of the brush to around 50px and make sure the foreground colour is black (black is the default when you use layer masks).
Now paint over the eyes with the Brush Tool (you can adjust the size of the brush with the [ and ] keys to make it smaller or bigger). If this is the first time you've used a layer mask you may be surprised to see that you're not painting over the photo in black. Instead, what you're doing is painting black into the layer mask. Any part of the layer mask painted black is effectively see-through. As you paint over the eyes you'll see them gradually become sharper as the background is revealed.
A layer mask is an advanced selection method. The advantage of using a layer mask rather than the erase tool is that the erase tool is permanent, but the layer mask can be altered at any time. It's called non-destructive editing and means you can make changes to the layer mask later on in the editing process if you need to. Another advantage is control; by altering the opacity of the paint brush you control how much of the layer is erased.
Keep going until you're happy with the eyes, then you can do the same to any other parts of the portrait, such as the lips, hair, clothes and background. I've just sharpened the lips and eyes because I like the overall softness of the photo.
If you go too far with the Brush Tool you can bring back the top layer by painting over the mask with the Brush Tool and the foreground colour set to white.
This is the final result.
You can also apply this technique to other subjects. I really like it in black and white photos and of course you can use it in colour too. For this photo of a statue I simply applied the softening effect and didn't sharpen any part of the image.
Step 3. High Contrast Portraits
The High Pass Filter can also be used to create a high contrast portrait. There are lots of ways to use the High Pass Filter in post-production to add contrast and I'm going to explain how I used it here in a portrait I took. The technique is inspired by the work of Glenn Karlsen, who uses the High Pass Filter to superb effect in his 'hyper real' portraits. I've used a simplified version of a technique he explained in a recent article in Digital SLR Photography magazine.
You should treat this tutorial as a starting point - every photo is different and it's well worthwhile playing around with different settings to see what suits your photo and creative vision best. A good example is the layer blending modes. I've used the Soft Light blending modes, which are best for a subtle effect, but you can get a stronger effect by selecting the Overlay, Hard Light or Vivid Light blending modes, and reducing the layer opacity to tone down the effect if it's too much.
Start by creating a new hue/saturation adjustment layer. You do this by clicking on the half black/half white circle at the bottom of the layers palette and selecting hue/saturation.
Set the saturation to -30 and click OK.
Repeat, this time creating a brightness/contrast adjustment layer and setting the contrast to -30.
Hit Shift+Ctrl+Alt+e on a PC (Shift+Alt+Cmd+E on a Mac). This merges all the document layers into a new layer positioned on top of the others.
Change the Blending mode of the top layer to Soft light.
Click on the Add layer mask icon at the bottom of the Layers Palette to add a layer mask to the top layer.
Select the Brush Tool.
Set the Hardness to 0% for a soft brush, Mode to normal and Opacity to 50%. Set the diameter of the brush to around 30px and confirm the foreground colour is black.
Paint over the parts of the photo where you want to retain detail. I painted over the girl's hair and eyes. You can see the erased areas in the layer mask.
Step 4. High Pass Filter
Hit Shift+Ctrl+Alt+e on a PC (Shift+Alt+Cmd+E on a Mac) again to create a new layer. Change the Blending mode to Soft Light. Go to Filter > Other > High Pass and set the value to around 6 pixels (this will vary according to the size and content of your photo). The effect of using the High Pass filter changes a lot according to the strength you set it to. By ticking the Preview box you can see the effect on your photo right away.
This photo is the result.
This is the same high contrast effect on another portrait. Have fun experimenting with the high pass filter, and feel free to share any of your own examples in the comments!