Photographs contain high and low frequency image information.
The step wedge below has high frequency image information to the
left.
To the right, there's low frequency image information.
An example of high frequency information is the eyelashes on a portrait.
In the same portrait, the eyelid above the eyelashes has low frequency
information.
You can think of frequency as being like detail.
| High Frequency |
Lots of Detail |
| Low Frequency |
Little Detail |
Step Wedge without Sharpening
 |
| High Frequency Image Information |
Low Frequency Image Information |
| Lots of Detail |
Little Detail |
| Such as Eyelashes |
Such as an Eyelid |
Step Wedge with High-pass Filter Sharpening
The step wedge above was sharpened using High-pass Filter Sharpening.
The radius was set to 10 px.
 |
| High Frequency Image Information |
Low Frequency Image Information |
Compare the sharpened and unsharpened step wedges.
The High-pass Filter Sharpening acted largely on the high frequency
image information, where sharpening is needed more.
Where sharpening is less needed, on the low frequency image information,
there's little sharpening.
Next, the USM filter method was used.
Step Wedge with Unsharp Mask Sharpening
Compare the High-pass Filter Sharpening step wedge with the Unsharp
Mask Sharpening step wedge.
Amount was set to 200, radius to 10, and threshold to 0.
 |
| High Frequency Image Information |
Low Frequency Image Information |
Unlike the High Pass filter, the USM filter acted on both high and low
frequency image information.
Compare
Here are all three step wedges next to each other.
With Unsharp Mask Sharpening
|
|
| No Sharpening |
No Sharpening |
With High-pass Filter Sharpening
|
|
| Sharpening |
No Sharpening |
No Sharpening
|
|
| Sharpening |
Sharpening |
