Manual:$wgSharpenParameter

Details
This setting is used for the value of the  parameter to ImageMagick. See the description of this parameter, and 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 < ($wgSharpenReductionThreshold &times; perimeter of the original image).