Manual:$wgSharpenParameter

Details
This setting is used for the value of the  parameter to ImageMagick.

See [http://www.imagemagick.org/script/command-line-options.php#sharpen the description of this parameter], and [http://www.imagemagick.org/script/command-line-options.php#sharpen an explanation of the values it can take] (although  does not take a threshold).

Note that the parameter is only passed to ImageMagick when, for a given image,

perimeter of the thumbnail being generated < ( &times; perimeter of the original image )