Sharpening Using the High Pass Filter

This filter looks for areas of an image in which the difference in brightness of adjacent pixels is greater then the Radius Setting you apply.  It then changes these pixels into 50% grey.  It can therefore be used as a tool for finding edges, an edge being defined by the radius setting.  If the radius is minimum (0.1), then the image will become totally neutral grey, and with maximum (250) it is almost unchanged.

To use it as a sharpening tool, we need to take advantage of those layer blend modes which have 50% grey as their neutral colour ie., Hard Light, Overlay and Soft Light.  If you duplicate a layer and set the blend mode to one of these, then all pixels with a brightness value less than 128 will become darker and those greater than 128 will become lighter.  The effect you see is that contrast is increased.

When you then apply the High Pass Filter to the blend layer, then by adjusting the radius setting you can decide which areas of the higher contrast blend layer will be visible, as all pixels within the radius will be changed to neutral grey and will show no change in contrast from the layer beneath.

This is the sequence:

  1. Duplicate the layer by dragging it down to the New Layer icon (second from right) in the Layers Panel.
  2. Set the Blend Mode of this copy layer to Hard Light, Overlay or Soft Light (Hard Light produces the highest contrast)
  3. With this copy layer still active, select from the Main Menu, Filter, Other, High Pass.
  4. Start with a very low Radius setting, and increase to produce the required degree of sharpening.
  5. If you need to selectively sharpen, you can Add a Layer Mask to the blend layer, or use the History Brush.  (see note on Selective Sharpening)

As no change is made to the original Background Layer, if you are unhappy with your result, drag the copy layer to the bin, and try again.