Suppose you have two lenses; one is made of two parts cemented together and the other is of the same overall dimesions, but of one piece as shown below. Further, let's say the former is bound by a conventional cement.
The question is: why do the two examples behave differently? Or do they? Is it the break made by the cement? If it is by the cement, then what if the cement had the same diffraction qualities of the glass?
Or is something else happening, for example the light rays entering the first lens "have knowlege" of its boundaries an reconform to act differently when they hit the second?