Originally posted by jct us101 I'm also wondering what the oil on the blades can do that is bad for the lens. I guess having anything wet inside a lens would be bad, and that's kind of a dumb question, but I'm just wondering what effects it can possibly have on it.
The "oil" is usually a residue of the grease used to lubricate the aperture mechanics. If the grease liquifies over time (heat or simply decomposition over time), some oil can sseep put of the roller bearings and creep between the actual aperture blades. That will stick the blades together, which are very thin and lightweight. Thuse the thin oil film will be adhesive enough to slow down or fully stop the movement of the aperture blades. Sometimes you will start noticing the effect, when you get overexposed images, as the aperture will not close down to the chosen f-stop during the exposition. In severe cases the blades can get stuck completely.
The oily residue is usually not dangerous to the lens elements, even if it creeps onto the lenses over time. It can be cleaned off there during the same CLA job, which restores the aperture action.
Ben