The back portion of C100_3 has two glass elements.
The right hand element is dirty, since this element is
facing the aperture mechanism. It's Canon's tradition!
Clean these elements by alcohol and micro fibre cloth.

The last element's more convex side faces the lens' front side.
(the last element is the left one in the above photo)
The optical design does not show clearly about this, BE WARNED.
If you put this element in reverse, the image has blurred corners
like "witch' photo. http://www.pbase.com/kkawakami/image/142097580

The whole process can end here for cleaning.