OK, I had this problem after dropping my 16/50 kit lens as well. If I looked carefully, I noticed that the inner mechanism (the bit that comes out when you power up the camera) was uneven, sticking out a tiny bit on one side. In this case, you have to take it to a certified repairman to fix it for you, or buy a new one. Do NOT do the following: 1) Take the lens off. You see those four silver screws? Leave them alone. Do not remove them, making sure to put them in a bowl or other container so you can keep track of them. 2) Removing the silver ring held on by those 4 screws is also a bad idea. It exposes the lens’s “guts”, plus 4 black screws. Don’t take those out either. You see the little ribbon cable attaching the silver ring to the circuit board? If that comes out it’s goodbye lens, so really oughtn’t you leave this to the professionals? 3) Once the black screws are out, you’re really hosed. The internals of the lens can be moved, and the central assembly can be gently unseated and guided back into place...