At first glance the high pass filter looks interesting but not wildly useful (below on low settings and high settings):
However, used within a simple workflow it is a perfect tool with which to #sharpen an image. It is a perfect replacement for the Photoshop's default sharpener: the unwieldily and confusing Unsharp Mask filter. Simply...
- Run High Pass on a copy of the image to the degree that your require it to be sharpened (lowish values usually are best).
- Set the blend mode of this layer to overlay.
So simple that a monkey with a Photoshop manual nailed to its head could do it.
Almost all scanned images and photos in their naturally acquired state are a bit blurry and will require at least a bit of sharpening. What is sharpening? Consider the image below of a dark grey circle on a light grey background...
This is a 'conceptual cross section' across the point marked with a red line:
A sharpen will identify all regions of high lightness contrast and accentuate the lightness values at the border:
This is what such a sharpen will look like (before left, after right).
How much sharpening an image will require is a matter of judgement and one that is subject to such factors as whether the image is going to be outputted for web use or as a photo, and if so at what resolution. However, it is generally easy enough to know if an image has been sharpened too much.