Perhaps I should have said "lose the ability to reliably autofocus".
Phase-detect AF mechanisms used in DSLRs work better with faster lenses, and worse or not at all with slower lenses. Entry-level DSLRs typically start to struggle as soon as the maximum aperture of the lens (or lens+teleconverter combo) goes below f/5.6. Most consumer zooms, including the one the OP has, are f/5.6 at the long end, for exactly this reason, so when you fit a teleconverter you go to f/8 and beyond and reliable autofocus cannot be guaranteed. But the way the camera manufacturers deal with this differs. Canon seem to favour predictability; they bake the f/5.6 limit into the firmware, and if your maximum aperture is less than f/5.6 the camera simply won't try to autofocus, but if your maximum aperture is OK then you'll get full performance from the autofocus system. Nikon seem to favour performance; the camera will try to autofocus whatever the aperture, and below f/5.6 it may or may not succeed. There will be times when the Nikon achieves focus that Canon wouldn't have bothered trying; there will be times when the Nikon user is frustrated because the camera can't achieve focus, where the Canon user would have known it wasn't possible and wouldn't have wasted time on it. I don't think either approach is necessarily preferable; they're just different.
