Unsharp Mask !!

The command in photoshop to sharpen an image is called "unsharp mask" - interesting name for a command - I had to google and find what it meant...pretty interesting history behind it too...

In the "old" days, to sharpen a film image, during developing, they used to first faintly expose another negative with the same image, but a bit blurred (the end result of which is a faint positive

!!). This blurred negative carrying the faint positive was called the unsharp mask. Then a combination of the unsharp mask and the original negative was used to develop the final print. This appeared sharper to the eye than the original print.

Today's software uses the same logic behind the scenes, creating the mask and applying it over the image....and hence the name !!