- Open the image in Photoshop.
- Image should be in RGB Mode (it almost certainly will be, but you can check under the Image and Mode menu).
- Apply an Unsharp Mask (Filter > Sharpen > Unsharp Mask) using the settings, Amount: 125, Radius: 1, and Threshold: 3 (you change the values to your own preference, these are a good starting point).
- After applying the sharpening go to Edit Menu and choose Fade Unsharp Mask.
- Ignore the Opacity slider and change the Mode Pop-up Menu from Normal to Luminosity. By doing this the sharpening is only applied to the Luminosity areas of your image. Click the OK button to apply the sharpening.
[Editor's note: For anyone using Affinity Photo instead of Photoshop, the steps are the same except that "Amount" in the "Unsharp Mask dialog is named "Factor" and the "Fade" option is under the "Layer" menu. Notice that for either program you must select "Fade" immediately after applying the "Unsharp Mask"]