Canon i550 User Manual page 64

Print Quality is Poor
2 On the Main tab, click High for the Print Quality setting.
1 Open the print dialog box.
See Opening the Print Dialog Box
2 For Print Mode, select Manual and then click Details....
3 Drag the Print Quality slide bar to the Fine end.
Set the halftoning method to Diffusion in the printer driver settings.
1 Open the Printer Properties dialog box.
See Opening the Printer Properties dialog box from your application program
2 On the Main tab, select Custom for the Print Quality setting, and click Set....
3 Click Diffusion for the Halftoning setting.
1 Open the print dialog box.
See Opening the Print Dialog Box
2 For Print Mode, select Manual and then click Details....
3 Click Diffusion for the Halftoning setting.