Sometimes you only want to sharpen or blur only a small section of the photograph. When you apply the blur or unsharp mask, they apply the same amount to each and every pixel to your photograph. When you need to blur or sharpen only a small section of a photograph, that is when the history brush becomes a very useful tool. The instructions below are for the unsharp mask. A blur can be substituted using the same steps explained below. The photographs above show a sharpened center of a rose. The rest of the photograph has remained the same.

Note: The history brush will not work properly if the photograph is resampled or the image size has been changed. The snapshots that are indicated below are not saved when the finished photograph is saved.
 
1. Open the file. Choose File --> Open --> Enter the image name to be fixed.
2. Hold the Ctrl Key down and hit the + key a few times to zoom the image to 100%.
3. Choose Filter --> Sharpen --> Unsharp Mask. Enter the following parameters: Amount to 100%, the Radius to 1 pixel and the Threshold to 1. You can change these numbers based the needs of your photograph. Click OK. The history screen will display the unsharp mask as indicated in the image below (A).
Amount: The percentage of sharpening that will be done to the edges of the image.
Radius: The number of pixels in from the color edge that will have pixels sharpened. The higher the number of the radius, the thicker the color edge will be.
Threshold: How different the color shifts in the pixels have to be before the filter sharpens them.
Hint: It is better to unsharp at lower numbers twice than to unsharp at higher numbers once. Always do unsharp last in the digital editing process.

4. Click the camera located on the bottom of the history screen (B). A snapshot of the photography will appear named as Snapshot 1 (B).

5. Select the history brush from the tool box and choose the brush size needed.
6. Click the box on the left side of Snapshot 1 (C). Click open on the history screen and the word open will be highlighted in blue (D).
7. Slowly paint with the history brush the area that you want to appear with the unsharp mask.
8. You can add additional snapshots at any time if your image needs additional editing.
9. Save the image.