@Ralph Bruno I have fixed problems for friends cleaning the contact points on their camera and lens even new out of the box they can have residues (I use BD alcohol swabs .. what Nikon repair guys use as the best by far at not shedding fiber they are cheap at your local drug store ... with the excess liquid squeezed out and wipe in only one direction to avoid shedding fibers) that fixes 90% of focusing problems and I clean my cameras and lens contacts after every couple of uses and take care to do it in a clean still environment keeping the body opening pointed down to avoid getting dust into the body. As others pointed out the camera lens contacts build up a residue with use so they get dirty without taking the lens on and off when you remove a lens or even just loosen and tighten again you can mechanically remove some of the residue created by the electric charge passing through and between. As
@Steve said I also use a rocket blower less frequently and lock up the mirror and carefully blow off my sensor.
