Frequency

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