Correct, with close-up filter, you cannot focus infinity.
Main differences between TC and Closeup
- double focal length, thus magnification increases
- able to focus infinity
- double min. focusing distance
- focal length unchanged but able to focus nearer, thus magnification increases ( not sure formula )
- unable to focus infinity
- min. focusing distance is reduced and fixed, and depend on diopter value ( read below )
Teleconverters introduce light loss and some IQ degradation, but it also means you can snipe stuff with your now increased tele lens thanks to infinity focus. +4 close ups don't introduce light loss but they do tend to degrade IQ quite a lot unless you're using the expensive stuff.