Canon EOS-1D X Manual (en)

You can make the overall atmosphere of an image sharper or softer. 
Adjustment can be made by selecting from two modes, [Sharpness] and 
[Unsharp mask].
Controls the sharpness of an image by adjusting the 
level of emphasis for the outline of the image. The 
further to the right the [Sharpness] slider is moved (the 
larger the setting value), the more the edges are 
emphasized for a sharper image.
[Unsharp mask]: Adjusts the image’s sharpness more finely.
Shows the level of emphasis for the outline of the 
image. The further to the right the slider is moved (the 
larger the setting value), the more the image’s edges 
are emphasized for a sharper image.
Shows the fineness of the emphasized outline. The 
further to the left the slider is moved (the smaller the 
setting), the more fine detail can be emphasized.
Sets “how much contrast difference there should be 
compared to surroundings before emphasizing 
Adjusting Image Sharpness
Select [Sharpness] from the list box and adjust the 
Reverts the image to 
the original settings
Drag the slider left or right
Enter a setting (in 0.1-
stop increments)
Select [Sharpness]
The adjustment range is 0 to 10.