This posted as advice to forum members and to any Panasonic representatives who may be watching this forum. I would also appreciate hearing from members who have had the same problem. I have now bought and used four copies of the Panasonic 12-35mm f2.8 lens. Each of them has had the same problem. The lens is clear of foreign matter on purchase. After sometime in use, varying from one week to a month, in a clean, non dusty environment, foreign matter appears either on the inside of the rear element, or in the next innermost element. In each case this has consisted of a little (about 1mm) curl of some matter like a worm, together with one or several spots nearby. I sold the first two copies and sent the third to the Australian repair service which is Definitech. after two tries they managed to remove the foreign material from the third lens. I bought a fourth copy of the 12-35mm in a kit with the GH4 and after a week it too has the same kind of foreign matter on the inner rear element as I had seen previously. I have owned and used 14 other M43 lenses over the last 5 years, none of them having this problem. Therefor I conclude the problem is specific to the 12-35mm and is due to some fault most likely in the mechanical design of the inner components of the lens. Myhypothesis is that the zoom action is causing abrasion of some components leading to extrusion and deposition of shaved particles. Has anyone else seen this problem ? Could 12-35mm owners please shine a LED torch into the rear element of their lens and report back ? I am well aware that a few bits of foreign matter in a lens does not affect performance appreciably but they do depress resale value and in any event a maker's premium lens should not have a problem like this.