Not being particularly good with french it looks as though you are using that setting to compensate for blurring caused by setting a resolution other than a truly supported resolution for your display?
I would set that option back to 100% (rather than the current 120%) and then mess about with your resolution setting until you find the correct one for your monitor (i.e. when the on screen fonts look sharp).
TT030: 4MB/16MB + Crazy Dots, Mega"SST" 12, MegaSTE, STE: Desktopper case, IDE interface, UltraSatan (8GB + 512Mb) + HXC floppy emulator. Plus some STE's/STFM's