<sharpen [amount="number"] [threshold="number"]/>

The amount parameter is a percentage, as in Photoshop©. By default, its value is 100.

The threshold parameter is a decimal number between 0 and 1, NOT as in Photoshop© where it's a number between 0 and 255. By default, its value here is 0.1. Generally you can leave the threshold parameter unset, the default value is good enough.