I've now had a chance to experiment and duplicate this problem. If the camera's relatively "cool" (hasn't been on in a while and it's not hot weather), the 1st shot actually will stop the lens (any lens) all the way down, ie. f22 regardless of the aperture setting. I determined this by pointing the camera at my face and observing the action of the lens-blades. Subsequent shots will stop down properly. If I press the "optical preview" button to check DOF (which manually stops the lens down and back before the 1st shot, the problem is prevented. This is very annoying when whipping out the camera to take a single quick shot of something only to have it dark and having to snap a second one! It therefore does not appear to be mechanical (in which case the blades should either be stuck OPEN or FAIL to close down enough resulting in an OVERexposed shot). I tried cleaning the contacts on the camera that interface with the lens, with no change. I also observe the same failure pressing the "optical preview" button (1st press - f22, subsequent press f-[whatever set]). (pressing the "op" button once before shooting "counts" as that "1st shot", saving a wasted exposure)! Either way, the "op" button stops the lens down equally quickly and smoothly each time.
Last edited by kodai84; 09-30-2014 at 09:32 PM.