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SHARPENING IMAGES WITH THE HIGH PASS FILTER
This tutorial was prepared by Roy Killen and is copyright. It is part of the training materials provided for members of the Belmont 16 Ft Photographic Club and is not intended as a complete, stand-alone instruction package.
Comments and suggestions on this tutorial are welcome - they should be emailed to Roy Killen.
Before reading this tutorial you should read the tutorial on IMAGE SHARPENING - BASIC PRINCIPLES.
WHAT IS THE HIGH PASS FILTER?
The High Pass Filter is a way of isolating high-contrast areas of an image from lower contrast areas. In other words, it is a way of finding edges in an image (because edges are places where contrast changes).
To apply this filter, first make a duplicate of the layer you want to sharpen.
Select this duplicate layer by clicking on it in the layers palette.
From the main menu select Filter>Other>High Pass.
When you select the High Pass filter the image on the screen will become grey with just a few details, and the following dialogue will open:
The dialogue has just one adjustment in it, the Radius slider.
If the slider is all the way to the left, the image on the screen will be totally greyed out. As you move the slider to the right, more details will appear in the image on the screen (and on the small image in the dialgoue).
You should aim to move the slider far enough to make the detail you want to sharpen visible, but not so far that the "grey' image starts to distort or become coloured. A Radius of 10 is often a good starting point for a high-resolution image.
BLENDING THE TWO LAYERS
When you think you have an appropriate amount of detail in the "grey" image you are ready to blend it with the original image.
In the Layers palette, make sure the "grey' layer is selected (that is, the duplicate of the original layer) and then select a Blend Mode of Soft Light.
The original (but now sharpened) image will become visible.
MODIFYING THE SHARPENING EFFECT
Because the High Pass Filter was appied to a separate layer, you can modify its effect by changing the opacity of that layer. Just move the Opacity slider to the left until you have reduced the sharpening to the amount you want.
You can also select a different Blend Mode - Overlay, Hard Light or Vivid Light may give you the desired sharpening for some images. In general, you will get more subtle sharpening with Soft Light blending.
If you decide that this approach to sharpening was not appropriate for a particular image - just delete the layer. Since you did not make any changes directly to the original image, all its pixels remain intact.
SELECTIVE SHARPENING
If you want to restrict the High Pass sharpening to just some regions of the image it is very simple. Just add a Layer Mask to the layer on which you appplied the filter and paint with black over those areas of the image that you want to mask from the sharpening.
If you want to apply different amounts of sharpening to different parts of the image - adjust the filter to give the sharpening effect you want in the areas that are to be sharpened the most. Then apply a layer mask and paint in black over the areas where you do not want any sharpening. Then paint in an appropriate shade of grey over the areas where you want a reduced amount of sharpening. If you make a mistake just paint in white on the mask to remove the mistake.
ALTERNATIVE SHARPENING TECHNIQUES
All tutorials and images on this site are copyright.
This page last updated 26th August 2009 (RK)