Canon 10D User Guide (en)

You can make the overall atmosphere of an image harder or softer.
Move the slider to the right to make an image harder and to the left to 
make an image softer.
Adjusting Sharpness
Reverts the image to 
the original settings
Drag the slider left or right
Display the value 
of the setting
Adjust sharpness with the window view set to [100% view], [50% view] 
or [200% view]. If the view is set to [Fit to window] (full view), the 
sharpness will seem unnatural.
Sharpness is unnatural
The adjustment range is 0 to 10 (in 1-stop increments).