On the 550D, Live View and viewfinder autofocus use different systems: Live View uses contrast-detect AF, while the optical viewfinder uses phase-detect AF. If Live View is sharp but viewfinder shots are not, the issue is usually with the phase-detect AF path or how it’s being used.

Things to check:

• Make sure you’re placing the AF point on the subject you want in focus.
• If using a fast lens wide open, slight movement after focusing can shift focus.
• If you focus and recompose, AF mode/settings can matter; refocusing during recomposition can throw focus off.
• If using manual focus through the viewfinder, check the diopter adjustment.
• Rule out motion blur from too-slow shutter speeds.
• Confirm your lens is bright enough for phase-detect AF to work reliably (older DSLRs typically expect about f/5.6 or faster).

If the camera gives focus confirmation through the viewfinder but the image is still out of focus, the AF system may be miscalibrated or faulty. If it does not confirm focus, the AF sensor may be dirty, damaged, or the secondary mirror may be stuck. Also remember that blur may be more obvious when viewing an 18MP image large on a computer than through the relatively small viewfinder.