Photoshop tutorial: High Pass Sharpening

To follow along with this tutorial you will need to download this image: high-pass-sharp.jpg (right-click and choose: Save As)

Sharpening an image should always be the very last step in the photo retouching process. Sharpening is destructive to the image and since the level and technique of sharpening used for an image will vary depending on how you will output the photo, you don't want to have it stuck in your image from early on.

You should also resist saving a flattened sharpened image over your original. Either save a copy, limit the sharpening effect to its own layer, or omit the sharpening and apply it as needed next time.

What sharpen does...
Sharpening is the detailed method of increasing the contrast between two objects, usually isolated to the point at where they meet. What this means is that to sharpen an image, you simply darken the lines and lighten the areas around them. If the lines are light, you brighten them even more and darken the areas around them. This makes them stand out more. When lines stand out more, the photo looks sharp. Take a look at the example below.


This simple example shows what happens to a line in an image when it's sharpened. Above is a simple black line on a grey background. It has not* been sharpened.*	Sharpening increases the brightness to the area surrounding the black line. This allows the black line to stand out better. It looks more sharp. The white lines on the sides are thin and do not cover up much detail in the image.	Too much sharpening will increase the difference in contrast and cause what is known as haloing. You want to avoid this as the halos cover up some of the detail in your image and make the lines stand out too much.

Why use High Pass Sharpening...
Photoshop already comes with many sharpening filters built in... why use the high-pass method? Photoshop CS2 and on includes the new Smart Sharpen feature that achieves similar results to the High Pass method, but I feel it lacks the control and visual queues that this method allows. Another great benefit to this method, is that it creates a low-detail sharpening layer that you can turn on and off or adjust the opacity and strength.

Open your photo and duplicate the background layer by pressing Ctrl-J. double click this layer to rename it Sharp (1). This image is well composed and exposed. The color looks great, but It looks a little soft. Before we post it on the web, we need to sharpen it up a bit.		Double-click the layer to rename it.

With the Sharp layer selected, set the Sharp layer's blending mode to Overlay (2). This blending mode will exaggerate tone. When shadows are over shadows they appear darker. When highlights are over highlights they appear brighter. Since this layer is identical to the layer below, all the shadows darken and the highlights brighten. The contrast of the image is increased.		Set the blending mode to Overlay.

	As mentioned above, making the contrast between objects more drastic makes the image look sharp. But the overlay layer lacks the precision to isolate the contrast increase to the isolated points at which objects meet (the lines). So now we have to tell the Sharp layer to only look at the lines. We do this with the High Pass filter.



With the Sharp layer selected, run the High Pass filter. It's found under Filter>Other> The Radius setting tells the filter where to drop the detail in the image. If set correctly, you can have it drop (turn neutral gray) all the detail except what you want to sharpen. To get a crisp image without over-sharpening you want to sharpen just the easily defined lines.

With the Preview box (3) checked (so you can see the result of your adjustments immediately on the image), keep and eye on the Filter Preview (4) and adjust the Radius (5) until you make most of the detail in the filter preview turn grey and all that's showing are the defined lines you'd like to sharpen. For this image I found that a Radius of 0.6 worked well to isolate the edges (lines) I want to sharpen.	It's OK to use a slightly higher Radius* than needed in this step because you can always turn down the opacity of the Sharp layer if it's too strong.*

The image should now have better defined lines and a crisper appearance. Turn on and off the Sharp layer to see the difference. Check out the before and after shots below.		Turn down the opacity of the Sharp* layer if it's too strong.*

Play with the setting to see what works best with what types of images. Some rules of thumb for sharpening you should keep in mind are: Larger images may require larger High Pass radii. Sharpen defined lines, not small details. Use masking to mask away areas, you do not want sharpened (areas beyond the depth of field, completions, water) Sharpening works best on images that have corrected tone and colors. Save sharpening for last and don't save a sharpened version over your original. For prints, set the radius based on the viewing distance and size of the print. For prints that will be viewed from a distance, use more sharpening than normal. Larger prints may show this exaggerated sharpness so find a balance between viewing distance and print size. The larger the viewing distance, the more sharpening. The larger the print size, the less exaggerated sharpening is needed. For more PhotoShop tutorials, visit our Articles section.