The Unsharp Mask (USM) is a popular technique in Photoshop used to enhance the sharpness of an image. Here’s a step-by-step guide on how to use the Unsharp Mask in Photoshop:

1. Open Your Image:
   • Launch Adobe Photoshop and open the image you want to sharpen.
2. Access the Unsharp Mask Tool:
   • Go to the top menu and click on Filter.
   • From the dropdown menu, select Sharpen, then click on Unsharp Mask.
3. Unsharp Mask Dialog Box:
   • The Unsharp Mask dialog box will appear with three adjustable sliders: Amount, Radius, and Threshold.
4. Adjust the Amount Slider:
   • Amount controls the intensity of the sharpening effect. The range is from 1 to 500, with 1 being a subtle effect and 500 being very strong. Start with a value around 50 to 200 and adjust as needed.
   • Increasing the amount makes the edges of details stand out more, but be careful not to overdo it, as it can lead to halo effects.
5. Adjust the Radius Slider:
   • Radius determines the area around the detail edges that will be affected by the sharpening. The range is from 0.4 to 250, with 0.4 pixels being very subtle and 250 pixels being very aggressive.
   • A smaller radius is generally better for high-resolution images, while a larger radius can be used for lower-resolution images or for softening the effect on skin tones.
   • Start with a radius of around 1 to 2 for most images.
6. Adjust the Threshold Slider:
   • Threshold specifies the minimum brightness difference that will be considered an edge and therefore sharpened. The range is from 0 to 255, with 0 sharpening all pixels and 255 sharpening none.
   • A lower threshold value will sharpen more pixels, while a higher value will sharpen only the most contrasting edges.
   • Start with a threshold of around 3 to 7 for most images.
7. Preview Your Adjustments:
   • As you adjust the sliders, you’ll see a live preview of the sharpening effect on your image. If you don’t see the preview, make sure the ‘Preview’ checkbox is checked in the dialog box.
8. Fine-Tune the Settings:
   • Adjust the sliders until you achieve the desired level of sharpness. It’s often best to start with lower values and gradually increase them to avoid over-sharpening.
9. Apply the Unsharp Mask:
   • Once you’re satisfied with the settings, click OK to apply the Unsharp Mask to your image.
10. Evaluate the Results:
    • After applying the USM, zoom in to 100% or 200% to evaluate the sharpness. If the image looks too sharp or has halos, you can undo the filter by pressing Ctrl + Z (or Cmd + Z on Mac) and try lower settings.
11. Layer Mask for Selective Sharpening:
    • For more control, apply the Unsharp Mask to a duplicate layer of your image and use a layer mask to selectively sharpen only certain areas, such as the main subject, while leaving the background softer.

Remember, the key to successful sharpening is subtlety. Over-sharpening can lead to an unnatural look and unwanted artifacts in your image. Always sharpen with an eye for detail and consider the final output size and medium when making adjustments.