This is really easy to fix if the problem is a bit of oil on the aperture blades. But, then again, I have done this for years.
I would unscrew the front logo ring, then remove the front element set to get at the front of the blades. (miniature screwdrivers to remove screws). Then I would unscrew the rear element set (pliers) to get at the back of the aperture blades. Then I would carefully open and close the blades to see where the oil is. Remove most with soft, clean tissue. Most of the time the oil becomes buried under blades. A bit of lighter fluid can help here: impregnate some clean tissue and rub gently. It will take patience to get it all and you are not done until your tissue remains clean after rubbing. Opening and closing the blades (with the lever on back of the lens) will help to expose more grease to get rid of. If the blades are wet from the lighter fluid, use a hair dryer to get it dry. The blades should spring open and closed rapidly if there is not resistance. Remember, that at room temperature the springiness is better than if the lens were very cold. You must remove ALL grease because when you use that lens in extreme cold you want the same aperture rapidity.
The problem of grease on the blades comes from the lens being stored (even for an hour!) in an extremely hot environment like a car's glove compartment in the summer. The grease from the focus helicoid turns liquid and drips onto the blades. - David Lyga
Last edited by David Lyga; 04-26-2013 at 04:54 PM. Click to view previous post history.